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Sulfonation of pyropolymeric fibers derived from phenol-formaldehyde resins
Carbon 40 (2002) 2323–2332
Sulfonation of pyropolymeric fibers derived from phenol-formaldehyde resins Kelly R. Benak a , Lourdes Dominguez a , James Economy a , Christian L. Mangun b , * a
Department of Materials Science, University of Illinois, 1304 W. Green St., Urbana, IL 61801, USA b EKOS Materials Corporation, 101 Tomaras Avenue, Savoy, IL 61874, USA Received 16 January 2001; accepted 18 March 2002
Abstract This research demonstrates that pyropolymeric fibers derived from phenol-formaldehyde resins can be successfully sulfonated providing advantages that are not currently available with traditional polymeric ion-exchangers. Benefits of this system include higher thermal stability, elimination of osmotic shock associated with beads, and enhanced ion-exchange capability. In this study, a series of thermally and chemically activated fibers were prepared by heating a phenolic precursor under inert atmosphere at temperatures between 350 and 600 8C. Subsequent sulfonation of the fibers in concentrated sulfuric acid incorporated sulfonic acid units as well as various oxygen-containing groups. The identification of functional groups and their thermal stability was carried out using elemental analysis (EA), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), and thermal gravimetric analysis (TGA). The effect of sulfonation on pore volume and surface area is discussed. Functional groups added during the sulfonation process serve as either strong acid or weak acid cationic exchangers. 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Char; B. Chemical treatment, Surface treatment; D. Chemical structure, Functional groups
1. Introduction Environmental pollution has become a topic of great concern to private citizens, industry, and governments throughout the world. This has led to stricter regulations for waste removal and tougher penalties for contaminating air or water sources [1]. In many cases, ion-exchange resins and granular activated carbons have been determined to facilitate the best available technologies for the removal of pollutants from waste streams and the remediation of contaminated sites. Ion exchange resins have been available commercially for almost sixty years and have been used extensively for the purification and demineralization of water [2]. These resins consist of three-dimensional covalent networks to which bound ions are attached. While ion-exchange beads can be very effective, it should be noted that they have a number of drawbacks. During the activation stage, solvents *Corresponding author. Tel.: 11-217-356-7162; fax: 11-217352-6655. E-mail address: [email protected] (C.L. Mangun).
must be used to pre-swell the cross-linked beads to facilitate sulfonation. The beads are very susceptible to fracture and breakage due to osmotic shock and must be kept wet at all times. The beads frequently require costly service containment systems. More importantly, there are a number of major needs with respect to environmental pollution where greatly improved ion-exchange materials are required. These needs include materials with very high contact efficiencies, a capability for rapid regeneration, and a much longer service life. Activated carbons have been primarily used for the removal of molecules, by physical adsorption, from waste streams. High surface area activated carbon fibers (ACFs) were first prepared in 1969 by direct carbonization and activation of a phenolic fiber in steam / CO 2 at 800– 1000 8C, and are commercially available through Nippon Kynol姠 [3,4]. The ability to tailor the micropore size and surface chemistry of the fibers has been shown to markedly enhance adsorption of trace contaminants from air and water [5–8]. Furthermore, the fibrous nature provides excellent contact efficiency with contaminated media, resulting in rapid adsorption and desorption rates [9].
0008-6223 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 02 )00146-X
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These unique properties, coupled with excellent chemical and thermal resistance, provided an incentive to investigate the preparation and evaluation of high surface area carbonaceous ion exchanging fibers. Oxidation of carbons to prepare weak acid cationic (WAC) exchangers has been extensively studied [6,10– 12]. The ability to functionalize carbons with strong acid cationic (SAC) groups would lead to systems with ionexchange capability over the entire pH range. The preparation of high surface area carbons typically involves treating precursors to high temperature (700–1100 8C) in activating environments [13]. Roman-Martinez et al. [14] reported that the degradation of phenolic resins at temperatures above 700 8C was associated with a loss of aromatic hydrogen. The presence of aromatic hydrogen is essential for electrophilic substitution reactions such as sulfonation. Due to the poor reactivity of carbons activated at temperatures above 700 8C, reports of sulfonation in the literature are scarce. Instances where it has been accomplished are for coals with sulfuric acid and carbon blacks with oleum, albeit with low ion-exchange capacities ranging from 0.1 to 0.79 mEq g 21 [15,16]. The scope of this work encompasses the preparation of SAC exchange fibers derived from phenolic resins. Principal variables investigated to maximize the degree of sulfonation included the carbonization and sulfonation temperatures. In order to retain the aromatic hydrogen necessary for sulfonation, phenolic fibers were thermally and chemically activated at significantly lower temperatures than those normally used [13]. The method for preparation of the cationic exchange fibers will be covered as well as the optimization of the activation process and characterization of end products (including ion-exchange capacity at low / high pH). The process of carbonization is not complete until the heat treatment temperature exceeds 1000 8C. Below this temperature, the materials have intermediate properties between polymer and carbon and are thus termed pyropolymers or, alternatively, chars [17]. Because the approaches described involve low temperature treatments (,600 8C), considerable retention of the organic functionality is observed in the infrared spectra and elemental compositions of the fibers. Thus, it is believed that the terms pyropolymer or char are more descriptive and appropriate than carbon for these systems.
2. Experimental
2.1. Preparation of ion-exchange fiber samples 2.1.1. Activation of fibers In this work, three sets of precursors to ion-exchange fibers were prepared. The first fiber set, derived from the phenolic fiber Kynol姠, has been described previously [18]. Kynol姠 fibers were heated under 400 ml min 21 dry
nitrogen flow at a desired temperature between 500 and 600 8C to make thermally activated phenolic fibers (TAPFs). The second set of fibers was prepared following a patent by Economy et al. [19]. Preweighed samples of Crane 232 chopped glass fibers were dipped into a phenolic resin solution (25 g of Novolac resin, 1.7 g of hexamethylenetetramine, and 100 ml of ethanol) and placed in a Lindberg tube furnace under dry nitrogen at a flow rate of 400 ml min 21 . The coated fiberglass was cured by first heating to 100 8C for 20 min to remove the solvent, then heating at 10 8C min 21 to 150 8C, and finally heat treating it isothermally at 150 8C for 20 min. The resultant fiber had a 61.4 wt% resin coating. In a separate experiment, the resin concentration in ethanol was reduced (12.5 g of Novolac resin, 0.85 g of hexamethylenetetramine, and 100 ml of ethanol) to produce thinner carbon coatings supported on the fiber substrate (38.0 wt%). The composite fibers were activated at 525 8C to prepare thermally activated phenolic coatings supported on glass fiber substrates (TAPG). With the third set of fibers, a chemical activation method developed by Yue et al. [20] was employed. Preweighed samples of Crane 232 chopped glass fibers were dipped into a phenolic resin solution (15 g of Novolac resin, 1.0 g of hexamethylenetetramine, 13.95 g of ZnCl 2 , and 100 ml of ethanol) and placed in a Lindberg tube furnace under dry nitrogen at a flow rate of 200 ml min 21 . The coated fiberglass was heated to 100 8C for 20 min to remove the solvent, then cured by heating at 10 8C min 21 to 150 8C and held isothermally for 20 min. The resultant fiber had a 55.9 wt% resin coating. The resin concentration in ethanol was reduced (7.5 g of Novolac resin, 0.5 g of hexamethylenetetramine, 13.95 g of ZnCl 2 and 100 ml of ethanol) in order to produce a thinner carbon coating supported on the fiber substrate (35.5 wt%). All of the composite fibers prepared in this way were activated under nitrogen flow at 350 or 450 8C to produce an activated coating on the glass fibers. The fiber was washed with distilled water followed by 0.1 N HCl three times to remove residual zinc compounds from the accessible pore structure. These fibers will be referred to as chemically activated phenolic coatings supported on glass fiber substrates (CAPG).
2.1.2. Functionalization Approximately 1 g of each of the activated fibers was functionalized in 150-ml concentrated sulfuric acid (Fisher Scientific, 98% purity by volume) for 3.0 h at temperatures ranging between 160 and 250 8C. The acid was preheated in a glass beaker with continuous stirring. Substantial evolution of bubbles was noted upon fiber addition and definite degradation occurred as confirmed by weight loss and a solution color change from clear to amber. The samples are denoted using the fiber type, the carbonization temperature, and the sulfonation temperature (e.g. TAPF525-160 is a thermally activated phenolic fiber carbonized
K.R. Benak et al. / Carbon 40 (2002) 2323 – 2332
at 525 8C followed by sulfonation at 160 8C). After sulfonation, the samples were rinsed thoroughly with 0.1 M NaHCO 3 solution to neutralize any residual acid. The ion-exchange sites in the fibers were then regenerated with 2 N HCl, rinsed thoroughly with distilled water, and dried for 12 h in air at 130 8C prior to ion-exchange capacity determinations.
2.2. Elemental analysis ( EA) Elemental analysis was performed using a model 240XA Elemental Analyzer (Control Equipment Corporation). The nitrogen, carbon, hydrogen, and sulfur contents were determined directly while the oxygen content was calculated by difference. For brevity, nitrogen and / or zinc contents were not reported since the amount of these elements was not significant (,1%).
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aliquot of the solution was removed and titrated with standardized 0.1 M sodium hydroxide solution using a phenolphthalein indicator. The volume of sodium hydroxide used, times the normality per gram of exchange material, is representative of the normalized loading capacity in mEq g 21 units. For a combined measurement of strongly and weakly acidic groups, 100 ml of 0.05 M sodium hydroxide solution (excess) and 0.5 g of sample were added to a bottle beaker. The contents were shaken for 4 h to ensure that the solution had come to equilibrium. The solution was then back titrated with standard 0.1 M hydrochloric acid solution using a phenolphthalein indicator. The mmol of hydrochloric acid subtracted from the mmol of initial sodium hydroxide per gram of exchange material is representative of the normalized loading capacity in mEq g 21 units.
2.6. Nitrogen and carbon dioxide adsorption 2.3. Diffuse reflectance fourier transform infrared spectroscopy ( DRIFTS) A Magna IR TM Spectrometer 550 (Nicolet) was used in reflectance mode to measure the infrared spectrum of the fiber samples and to identify their chemical functionalities. Data acquisition was performed automatically using an interfaced computer and a standard software package (Omnic). The samples were dried under vacuum at 150 8C prior to mixing with KBr powder (|10 wt% carbon). The samples were run in ratio mode allowing for subtraction of a pure KBr baseline. The sample chamber in the spectrometer was purged with nitrogen (20 min) to remove carbon dioxide and water. A total of 64 scans were then taken for each sample at a resolution of 4 cm 21 using triangular apodization to smooth the noise level.
2.4. Thermal gravimetric analysis ( TGA) All thermal gravimetric analyses were obtained using a TGA 951 (TA Instruments, New Castle, Delaware) interfaced with a TA Instruments 2100 computer. The overall thermal stability of functional groups added through sulfonation was monitored by heating the samples at 10 8C min 21 to 800 8C in an inert nitrogen atmosphere. Coating contents of impregnated glass samples were determined from the mass loss of the sample due to combustion using a heating rate of 10 8C min 21 to 700 8C in air. The gas flow rate was 100 ml min 21 and the approximate weight of individual samples was 50 mg.
2.5. Ion exchange capacity measurements The cationic exchange capacity of strongly and weakly acidic groups was determined in a conventional manner [21]. For strongly acidic groups, 50 ml of 1 M sodium chloride solution (excess) and approximately 0.5 g fiber were added to a bottle beaker and stirred for 4 h. A 25-ml
A Quantachrome Autosorb-1-MP (Boynton Beach, FL) was used for the volumetric measurement of nitrogen and carbon dioxide adsorption isotherms. The adsorption temperature for nitrogen was 77 K while the adsorption temperature for carbon dioxide was 298 K. All samples were degassed for 3 h under vacuum at 130 8C. The adsorption experiments were performed in static mode using a manifold set to supply an adjustable dose of adsorbate to the sample cell. After equilibration, the pressure was recorded and the process was repeated at higher dose pressures until the saturation pressure was reached. The pressure difference between the previous pressure measurement and the next equilibrated pressure along with the dose volume was used to calculate the amount of adsorbate uptake by the sample. Micropore surface area and micropore volumes were calculated from the experimental CO 2 isotherm in the appropriate P/Po range using the Dubinin–Radushkevich (DR) equation. The total pore volume was estimated from the amount of nitrogen adsorbed at P/Po 5 0.95, assuming a liquid density of nitrogen at 77 K of 0.808 g cm 23 . The t-plot method, developed by Lippens and de Boer, was used to estimate the micropore volume from nitrogen isotherms.
3. Results and discussion
3.1. Ion-exchange fibers derived from TAPF 3.1.1. Characterization of surface chemistry of sulfonated fibers Results from elemental analysis, in conjunction with infrared spectroscopy, were used to characterize the surface chemistry of TAPFs prior to and after sulfonation. Conley et al. [22] have extensively studied the thermal degradation mechanism of phenolic resins. High temperature treatment (.500 8C) under inert atmospheres led to
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Fig. 1. Infrared spectra of TAPF series before sulfonation: (a) TAPF-600, (b) TAPF-575, (c) TAPF-550, (d) TAPF-525.
the reduction of phenolic hydroxyl functionality and an extensive oxidation from within the resin to form quinone and acid fragments on the surface. Infrared and elemental analysis results collected of samples prior to sulfonation were consistent with these early reports. Infrared spectra of all TAPFs directly after carbonization are depicted in Fig. 1. Reduction of phenolic hydroxyls (3546 cm 21 ) was observed as the heat treatment temperature was increased. Absorptions at 1683 and 1709 cm 21 indicated the presence of carbonyl groups. The absorbance at 1600 cm 21 is found with all oxidized carbons and was attributed to C=C stretching of polyaromatic systems. Furthermore, the absorbance at 2914 cm 21 decreased with respect to that at 3049 cm 21 indicating a change from aliphatic CH linkages to aromatic CH bonds. Morterra et al. [23] assigned absorbances in the range of 1150–1360 cm 21 as aromatic C–O stretches and phenolic OH deformation. Other peaks including 1462, 881, 814, and 748 cm 21 can be attributed
to CH bending and deformation. From elemental analysis results listed in Table 1, H / C and O / C mole ratios decreased as carbonization temperatures increased, indicating a greater loss of hydrogen and oxygen at higher heat treatment temperatures. After sulfonation, elemental analysis and infrared spectra indicated significant changes in the surface chemistry of the fibers. Regardless of the carbonization temperature, the sulfonation process incorporated sulfur and increased the oxygen contents significantly, with greater increases observed for the higher sulfonation temperature (Table 1). The increase in oxygen was significantly higher than expected as calculated from the addition of only sulfonic acid groups. Thus, it was concluded that oxidation of the microporous carbon accompanied the high temperature sulfonation of these systems. This result is consistent with reports in the literature [24]. By examining either the 160 or 220 8C sulfonation temperature, increases in the S / C and O / C ratios were correlated with decreasing carbonization temperatures. This indicated an increase in reactivity of the structure at lower carbonization temperatures. Infrared spectra of a carbonized, sulfonated, and sulfonated sample treated with sodium hydroxide (TAPF-600 series) are shown in Fig. 2. After sulfonation, the presence of characteristic –HSO 3 absorption bands at 1034 and 605 cm 21 assigned to S=O stretching and C–S stretching, respectively, indicates incorporation of SO 3 H groups into the structure. Elemental analysis showed that significant oxidation of the structure accompanied sulfonation, an observation that was also supported by infrared spectra. Puri [25] determined that three groups (carboxylic acids, quinones, and phenolic hydroxyls) typically dominate an oxidized pore surface. In agreement with this observation, the band at 3541 cm 21 extended and broadened to incorporate lower wavenumbers. This was attributed to the formation of carboxylic acids, which in the solid phase exhibit a broad band at 3300–2500 cm 21 due to OH stretching. The absorption at 1740 cm 21 increased con-
Table 1 Elemental analysis of TAPF series Sample
Wt% Carbon
Wt% Hydrogen
Wt% Oxygen
Wt% Sulfur
H/C mol ratio
O/C mol ratio
S/C mol ratio [310 3 ]
TAPF-525 TAPF-525-160 TAPF-525-220 TAPF-550 TAPF-550-160 TAPF-550-220 TAPF-575 TAPF-575-160 TAPF-575-220 TAPF-600 TAPF-600-160 TAPF-600-220
90.0 72.8 65.0 91.2 74.5 67.7 92.4 76.0 70.4 93.8 78.5 72.1
4.29 1.81 1.99 4.18 1.78 1.96 4.06 1.74 1.92 3.95 1.71 1.89
5.72 21.8 27.5 4.6 20.6 25.8 3.50 19.4 24.2 2.38 17.9 23.0
0.0 3.62 5.57 0.0 3.17 4.5 0.0 2.78 3.49 0.0 1.92 3.04
0.57 0.30 0.37 0.55 0.29 0.35 0.53 0.27 0.33 0.51 0.26 0.31
0.05 0.22 0.32 0.04 0.21 0.29 0.03 0.19 0.26 0.02 0.17 0.24
0.0 19 32 0.0 16 25 0.0 14 19 0.0 9.2 16
K.R. Benak et al. / Carbon 40 (2002) 2323 – 2332
Fig. 2. Infrared spectra of TAPF-600-220: (a) sample prior to sulfonation, (b) H 1 form, (c) Na 1 form.
siderably and was assigned to carbonyl stretching from carboxylic acid groups. Chemical modification of the carboxylic acid groups to carboxylic acid salts further confirmed the presence of these groups in the sulfonated fibers. Sodium hydroxide solution was used to exchange the hydrogen ions of the sulfonic acid groups for sodium, resulting in a decrease in intensity of the 1740 cm 21 band and the broad band centered at 3500 cm 21 . A new band at 1385 cm 21 appeared and is due to the stretching of carboxylate groups. Fig. 3 shows infrared spectra of the entire TAPF series after sulfonation at 160 8C. The sample carbonized at 600 8C had a very noticeable decrease in the ratio of absorbance of S=O with respect to phenolic OH deformation, indicating a relative decrease in the amount of sulfonic acid units.
Fig. 3. Infrared spectra of TAPF series after sulfonation at 160 8C: (a) TAPF-600-160, (b) TAPF-575-160, (c) TAPF-550-160, (d) TAPF-525-160.
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3.1.2. Thermal stability of sulfonated fibers The thermal stability of these materials is directly dependent on the decomposition temperature of the various functional groups. For example, the sulfonic acid group (–HSO 3 ) thermally decomposes near 250 8C, while oxygen containing functionality such as carboxylic acids, quinones, and phenolic hydroxyls have a higher thermal stability [24]. It is generally accepted that CO 2 is given off as a volatile from oxidized carbon surfaces at #350 8C mainly due to the decomposition of carboxylic acid groups, whereas the loss of CO is derived from the decomposition of phenolic hydroxyls and quinones at higher temperatures [26,27]. The thermal degradations of sample TAPF-600 prior to and after sulfonation were examined and the superimposed plots of mass change as a function of temperature are presented in Fig. 4a. The weight loss as a function of temperature provides an indication of the functional groups existing prior to or incorporated by the sulfonation treatment, which are decomposed as volatiles during heat treatment. Before sulfonation, the sample lost only 13.3% while the sulfonated sample lost 59.5% of its initial mass upon heating to 800 8C (this value does not include the initial loss, attributed to strongly adsorbed water). Thus, sulfonation increased the weight loss associated with functional groups by 4.5 times. Differential thermogravimetry (DTG) was used for thermal stability analysis of functional groups on sample TAPF-600 prior to and after sulfonation (Fig. 4b). The DTG curve was used to estimate the initial and final degradation temperatures for the sample [28]. Before sulfonation, the fiber was thermally stable up to 491 8C while the sulfonated sample began to lose discernible weight at 230 8C due to sulfonic acid groups followed by additional degradation due to the various acidic functional groups as described above. Even commercially available cationic exchange beads specifically designed with high thermal stability have a maximum use temperature of only 130 8C [29]. Therefore, these systems may find use in many industrial applications including the sorption of cations from corrosive media, hot solutions or melts. 3.1.3. Ion-exchange capacity of sulfonated fibers Elemental and FTIR analysis have confirmed that the sulfonation process created sulfonic acid functionality that can effectively exchange cations at all pH values. Fig. 5 shows the capacity, due to strongly acidic sulfonic acid groups, as a function of carbonization and sulfonation temperatures. The data shows that at higher carbonization temperatures the reactivity of the fiber for sulfonation decreases, possibly due to deactivation of the ring structure. On the other hand, capacity increased significantly as sulfonation temperature increased. As discussed previously, it was found that oxidation of the microporous carbon accompanied the high temperature sulfonation of these systems, creating carboxylic acid and phenolic hydroxyl functionalities that further enhance the exchange capacity
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Fig. 5. Strong acid ion exchange capacities of TAPF series as a function of carbonization and sulfonation temperatures.
cially available beads (Purolite C100X10H) was measured to be 4.7 mEq g 21 after the added step of an overnight pre-swell before testing.
3.1.4. Effect of sulfonation treatment on pore structure This work shows a correlation between the extent of functionalization, pore volume, and surface area and serves to demonstrate how sulfonation affects the porous structure of the fibers (Table 2). No significant nitrogen adsorption at 77 K was observed for samples thermally activated at temperatures below 600 8C. Daley et al. [18] determined that the porosity in thermally activated phenolic systems, in the absence of external etching agents, is due solely to the volatilization of degradation by-products. In that instance, the structure was directly imaged using STM and ˚ with the micropores were measured to be less than 11 A, Fig. 4. (a) Thermogram depicting weight loss due to decomposition of functional groups from carbon surface for (a) TAPF-600 and (b) TAPF-600-220. (b) Differential thermogram depicting decomposition peaks for (a) TAPF-600 and (b) TAPF-600-220.
of these materials at higher pH. Measuring the total ionexchange capacity of the material and subtracting the contribution from the strong acid group exchange determined the capacity of the weakly acidic functional groups. The data shows that, as the carbonization temperature increases, the capacity due to weakly acidic functionality remains stable (Fig. 6). Oxidation, unlike sulfonation, is not a substitution reaction requiring aromatic hydrogen from the carbon; therefore, higher carbonization temperatures do not affect reactivity in the same manner. In a similar fashion to the sulfonic acid groups, the concentration of oxygen groups increased with increasing functionalization temperature. Total ion-exchange capacities as high as 9.9 mEq g 21 were measured for the sulfonated fibers. For comparison, the exchange capacity of commer-
Fig. 6. Weak acid ion exchange capacities of TAPF series as a function of carbonization and sulfonation temperatures.
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Table 2 Surface area and pore volume for TAPF series Sample
Micropore volume [cm 3 g] 21 (CO 2 , 298 K)
DR surface area [m 2 g] 21 (CO 2 , 298 K)
TAPF-525 TAPF-525-160 TAPF-525-220 TAPF-550 TAPF-550-160 TAPF-550-220 TAPF-575 TAPF-575-160 TAPF-575-220 TAPF-600 a TAPF-600-160 TAPF-600-220
0.22 0.12 0.15 0.25 0.15 0.18 0.28 0.18 0.21 0.29 0.19 0.21
500 285 353 571 348 416 648 417 485 670 431 499
Properties calculated from nitrogen adsorption isotherm: micropore volume50.24 cm 3 g 21 , total pore volume50.27 cm 3 g 21 . Nitrogen adsorption is diffusion limited for all other samples. a
˚ Daley et al. the majority of micropores being less than 6 A. [18] also observed small mesopores on the surface ranging ˚ When nitrogen at 77 K is used as the in width up to 50 A. adsorbate to characterize essentially microporous solids, it may present a problem because of limited diffusion rates [30]. To overcome this problem, all samples were characterized using carbon dioxide since it can be carried out at room temperature thus facilitating its entry into the narrow micropores. After thermal activation at 600 8C, the total pore volume is quite close to the micropore volume from carbon dioxide adsorption because the above constrictions are removed and the microporosity is accessible to both adsorbates. From Table 2, sulfonation treatments decreased both the CO 2 micropore volume and surface area of the fibers, presumably due to the added amount of functional groups. Higher sulfonation temperatures resulted in a slight upswing of surface area, undoubtedly due to the additional etching of the structure that accompanies more aggressive treatment conditions. This is a similar finding to those of other investigators studying the effect of nitric acid oxidation on activated carbon pore structure [6,10]. Samples thermally activated at 600 8C that were subsequently sulfonated lost their capacity to adsorb nitrogen at 77 K. The sulfonic acid groups must add to the mesoporous regions found at pore openings, reported by Daley et al. [18], essentially blocking or significantly slowing nitrogen adsorption. Menendez et al. [31] also observed preferential
addition of functional groups to the external surfaces of carbons due to diffusional limitations of their oxidizing agents.
3.2. Ion-exchange fibers derived from TAPG Thermally activated phenolic coatings on glass were prepared using the optimized conditions determined for thermally activated phenolic fibers previously described (TAPF-525-220) in order to assess any benefits that might be associated with glass fiber supported coatings. Prior to sulfonation, the carbon coatings had identical structures and properties to those seen with thermally activated phenolic fibers. The FTIR, elemental analysis, surface area and micropore volume (normalized per gram of carbon), showed no significant differences and were excluded for brevity. Thus, the only variable of concern relates to coating thickness / carbon loading and its effect on ionexchange properties. In this study, impregnation loadings of 28.0 and 49.0 wt% (Table 3) were utilized. It was determined that high loadings produced results similar to those of the TAPFs. However, with thinner coatings, the capacity per gram of carbon increased for the strongly acidic functional groups due to the increased surface to volume ratio. This supports the notion that the sulfonic groups are primarily located near the mesopore openings of the fibers. In addition, elemental analysis confirmed
Table 3 Properties of ion-exchange fibers derived from TAPGs Sample
TAPG-525-220 TAPG-525-220
Coating (wt%)
28.0 49.0
Capacity [mEq g 21 coating] Strongly acidic
Weakly acidic
2.5 1.8
8.1 8.3
S/C mol ratio [310 3 ] 50 33
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these findings as the sulfur to carbon ratio increased for the thinner coatings. An interesting point to note is that the capacity due to weakly acidic functional groups is similar regardless of coating thickness.
Table 5 Coating percentage, surface area, and pore volume of CAPG series Sample
Coating (wt%)
Micropore volume [cm 3 g 21 coating] (CO 2 , 298 K)
DR surface area [m 2 g 21 coating] (CO 2 , 298 K)
CAPG-350 a CAPG-350-160 CAPG-350-190 CAPG-350-220 CAPG-350-250 CAPG-450 a CAPG-450-160 CAPG-450-190 CAPG-450-220 CAPG-450-250
53.4 61.8 51.4 48.0 42.0 48.2 49.0 47.6 46.8 41.9
0.44 0.18 0.22 0.28 0.31 0.59 0.46 0.50 0.57 0.64
1023 408 517 661 724 1375 1057 1170 1322 1485
3.3. Ion-exchange fibers derived from CAPG 3.3.1. Characterization of surface chemistry /pore structure Comparison of thermally and chemically activated systems shows that they have similar H / C ratios (Table 4) resulting in equivalent amounts of sulfonic acid groups; however the number of acidic functional groups is greatly increased for the CAPG. Prior to sulfonation, both chemically activated fibers (CAPG-350 / 450) have higher surface areas (versus TAPF) as calculated from CO 2 adsorption (Table 5). As discussed previously, the sulfonic acid groups are confined to the external surface of the fibers, thus leaving a greater number of sites to accommodate the weak acidic groups. Teng et al. [32] have shown that chemical activation of phenolic resins resulted in larger average pore sizes and broader pore size distributions than physical activation processes. This is supported by noting the difference between the micropore volume from CO 2 and the total pore volume from nitrogen, which indicates a shift towards mesopores. In addition, increased surface area and pore volume retention is observed after sulfonation, with more aggressive conditions leading to better retention. This suggests that the carbonaceous structure is more reactive to sulfuric acid resulting in increased etching to offset the weight gain caused by functional groups. 3.3.2. Ion-exchange properties Figs. 7 and 8 display the strong and weak ion-exchange capacities of the CAPGs. Note that the strong acid capacities of the CAPGs and the TAPGs are comparable due to their similar amounts of sulfonic groups, although sulfonation treatment at a higher temperature of 250 8C for the CAPG did achieve a better ion-exchange capacity. On the other hand, the weak acid capacity was dramatically
a
Properties calculated from nitrogen adsorption isotherm: CAPG-350 micropore volume50.36 cm 3 g 21 , total pore volume5 0.54 cm 3 g 21 , CAPG-450 micropore volume50.53 cm 3 g 21 , total pore volume50.73 cm 3 g 21 .
Fig. 7. Strong acid ion exchange capacities of CAPG series as a function of carbonization and sulfonation temperatures.
Table 4 Elemental analysis of CAPG series Sample
Wt% Carbon
Wt% Hydrogen
Wt% Oxygen
Wt% Sulfur
H/C mol ratio
O/C mol ratio
S/C mol ratio [310 3 ]
CAPG-350 CAPG-350-160 CAPG-350-190 CAPG-350-220 CAPG-350-250 CAPG-450 CAPG-450-160 CAPG-450-190 CAPG-450-220 CAPG-450-250
90.6 66.4 57.1 45.0 38.1 94.2 82.3 73.5 61.5 49.4
3.75 2.18 2.47 2.85 3.01 3.35 1.58 1.88 2.29 2.71
5.64 29.2 36.9 47.0 52.2 2.42 14.7 22.4 32.7 43.4
0.0 2.27 3.52 5.12 6.72 0.0 1.44 2.27 3.52 4.48
0.50 0.39 0.52 0.76 0.95 0.43 0.23 0.31 0.45 0.66
0.05 0.33 0.48 0.78 1.03 0.02 0.13 0.23 0.40 0.66
0.0 13 23 43 66 0.0 6.6 12 21 34
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fibers and the application of these systems for heavy metal removal from contaminated liquids.
Acknowledgements We would like to thank NSF (Grant [ DMR 97-12489) for supporting this research. We would also like to acknowledge and thank Joe Hayes of American Kynol Inc. for his continuing support of this program and for providing the Kynol fabric, Jan Nimrick and Rudiger Laufhutte in the Microanalysis Laboratory for elemental analysis results, and Crane & Company for kindly providing glass fiber substrates. Purolite provided the ion-exchange beads.
Fig. 8. Weak acid ion exchange capacities of CAPG series as a function of carbonization and sulfonation temperatures.
increased due to the incorporation of more functional groups into the porous structure. Total ion-exchange capacities as high as 17 mEq g 21 were measured, these being much greater than any values reported in the literature. In a similar fashion to the TAPGs, strongly acidic ion-exchange capacities were enhanced through thinner coatings due to the increased surface to volume ratios of the fibers (CAPG-350-250, 29.9% coating, 2.8 mEq g 21 ). This further confirms that the sulfonic acid groups are concentrating at the surface of the fibers, even with chemically activated samples of higher initial surface area.
4. Conclusions Strong acid and weak acid cationic exchange groups were incorporated into thermally and chemically activated phenolic fibers during sulfonation. The amount and type of functional groups can be varied by careful control of the precursor fiber, carbonization temperature, and sulfonation temperature. The ion-exchange capacity of the fibrous systems is competitive with those of cationic exchange beads at low pH and substantially better with higher pH influents. This characteristic can be very useful in industrial practice, since hard water is naturally buffered at pH .7. Material durability is equally important as capacity in ion-exchange processes. Use of carbonaceous ionexchangers eliminates several problems encountered with ion-exchange beads. For example, the fibers will not swell and therefore will not undergo osmotic shock. Secondly, the fibers are more thermally stable and do not lose significant weight at temperatures below 250 8C, which is the degradation temperature of sulfonic acid groups. Future studies will compare reaction rates of the beads versus
References [1] US EPA. US Environmental Protection Agency Home Page, Web page accessed February, 2000, Available at: http: / / www.epa.gov. [2] Dorfner K. Ion exchangers. Ann Arbor: Ann Arbor Science Publishers, 1972. [3] Economy J, Clark RA. Fibers from Novolacs. US patent 3650102, 1972. [4] Economy J, Clark RA. Method for production of Novolac fibers. US patent 3723588, 1973. [5] Economy J, Foster KL, Andreopoulos AG, Jung J. Tailoring carbon fibers for adsorbing volatiles. CHEMTECH 1992;22(10):597–603. [6] Mangun CL, Benak KR, Daley MA, Economy J. Oxidation of activated carbon fibers: effect on pore size. Surface chemistry, and adsorption properties. Chem Mater 1999;11(12):3476–83. [7] Mangun CL, Benak KR, Economy J, Foster KL. Surface chemistry, pore sizes and adsorption properties of activated carbon fibers and precursors treated with ammonia. Carbon 2001;39(12):1809–20. [8] Mangun CL, DeBarr JA, Economy J. Adsorption of sulfur dioxide on ammonia treated activated carbon fibers. Carbon 2001;39(11):1689–96. [9] Daley MA, Mangun CL, Economy J. Low-cost activated carbon fiber assemblies for control of contaminants from air / water. In: Proceedings of the 89th annual meeting of the Air and Waste Management Association. Nashville, TN (June), 1996. [10] Barton SS, Evans MJB, MacDonald JAF. Oxidized BPL carbons. Adsorp Sci Technol 1993;10:75. [11] Jia YF, Thomas KM. Adsorption of cadmium ions on oxygen surface sites in activated carbon. Langmuir 2000;16:1114– 22. [12] Johnson Jr. JS, Westmoreland CG, Sweeton FH, Kraus KA, Hagaman EW, Eatherly WP, Child HR. Modification of cation-exchange properties of activated carbon by treatment with nitric acid. J Chromatogr 1986;354:231–48. [13] Ruthven DM. Principles of adsorption and adsorption processes. New York: Wiley, 1984. [14] Roman-Martinez MC, Cazorla-Amoros D, Linares-Solano A,
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K.R. Benak et al. / Carbon 40 (2002) 2323 – 2332 Salinas-Martinez de Lecea C, Atamny F. Structural study of phenolformaldehyde char. Carbon 1996;34(6):719–27. Rivin D, Aron J, Richards LB. Sulfonated carbon blacks. US patent 3442679, 1969. Mittal AK, Venkobachar C. Uptake of cationic dyes by sulfonated coal: sorption mechanism. Ind Eng Chem Res 1996;35:1472–4. Jenkins GM, Kawamura K. Polymeric carbons-carbon fibre, glass and char. London: Cambridge University Press, 1976. Daley MA, Tandon D, Economy J, Hippo EJ. Elucidating the porous structure of activated carbon fibers using direct and indirect methods. Carbon 1996;34(10):1191–2000. Economy J, Daley M. Coated adsorbent fibers. US patent 5834114, 1998. Yue ZR, Mangun CL, Economy J. Preparation of fibrous porous materials by chemical activation. I. ZnCl 2 activation of polymer coated fibers. Carbon 2002:submitted for publication. Kunin R. Elements of ion exchange. New York: Reinhold, 1960. Jackson WM, Conley RT. High temperature oxidative degradation of phenol-formaldehyde polycondensates. J Appl Polym Sci 1964;8:2163–93. Morterra C, Low MJDIR. Studies of carbons. VII. The pyrolysis of a phenol-formaldehyde resin. Carbon 1985;23(5):525–30. Ermolenko IN, Lyubliner IP, Gulko NV. Chemically modified carbon fibers and their applications. New York: VCH Press, 1990.
[25] Puri BR. In: Walker Jr. PL, editor, Chemistry and physics of carbon, vol. 6, New York: Marcel Dekker, 1970, pp. 191– 275. [26] Moreno-Castilla C, Ferro-Garcia MA, Joly JP, BautistaToledo I, Carrasco-Marin F, Rivera-Utrilla J. Activated carbon surface modifications by nitric acid. Hydrogen peroxide, and ammonium peroxydisulfate treatments. Langmuir 1995;11:4386. [27] de la Puente G, Pis JJ, Menendez JA, Grange P. Thermal stability of oxygenated functions in activated carbons. J Anal Appl Pyrolysis 1997;43:125–38. [28] Chetan MS, Ghadage RS, Rajan CR, Gunjikar VG, Ponrathnam S. Thermolysis of orthoNovolacs. Part 1. Phenolformaldehyde and m-cresol-formaldehyde resins. Thermochim Acta 1993;228:261–70. [29] Sybron. Sybron Chemicals resin select page, Web page accessed in January, 2000, Available at: http: / / 63.241.177.140 / select.htm[Purolite. [30] Rodriguez-Reinoso F, Lopez-Gonzalez JD, Berenguer C. Activated carbons from almond shells. I. Preparation and characterization by nitrogen adsorption. Carbon 1982;20(6):513–8. [31] Menendez JA, Illan-Gomez MJ, Leon y Leon CA, Radovic LR. On the difference between the isoelectric point and the point of zero charge of carbons. Carbon 1995;33(11):1655– 9. [32] Teng H, Wang S. Preparation of porous carbons from phenolformaldehyde resins with chemical and physical activation. Carbon 2000;38:817–24.
Sulfonation of pyropolymeric fibers derived from phenol-formaldehyde resins Kelly R. Benak a , Lourdes Dominguez a , James Economy a , Christian L. Mangun b , * a
Department of Materials Science, University of Illinois, 1304 W. Green St., Urbana, IL 61801, USA b EKOS Materials Corporation, 101 Tomaras Avenue, Savoy, IL 61874, USA Received 16 January 2001; accepted 18 March 2002
Abstract This research demonstrates that pyropolymeric fibers derived from phenol-formaldehyde resins can be successfully sulfonated providing advantages that are not currently available with traditional polymeric ion-exchangers. Benefits of this system include higher thermal stability, elimination of osmotic shock associated with beads, and enhanced ion-exchange capability. In this study, a series of thermally and chemically activated fibers were prepared by heating a phenolic precursor under inert atmosphere at temperatures between 350 and 600 8C. Subsequent sulfonation of the fibers in concentrated sulfuric acid incorporated sulfonic acid units as well as various oxygen-containing groups. The identification of functional groups and their thermal stability was carried out using elemental analysis (EA), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), and thermal gravimetric analysis (TGA). The effect of sulfonation on pore volume and surface area is discussed. Functional groups added during the sulfonation process serve as either strong acid or weak acid cationic exchangers. 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Char; B. Chemical treatment, Surface treatment; D. Chemical structure, Functional groups
1. Introduction Environmental pollution has become a topic of great concern to private citizens, industry, and governments throughout the world. This has led to stricter regulations for waste removal and tougher penalties for contaminating air or water sources [1]. In many cases, ion-exchange resins and granular activated carbons have been determined to facilitate the best available technologies for the removal of pollutants from waste streams and the remediation of contaminated sites. Ion exchange resins have been available commercially for almost sixty years and have been used extensively for the purification and demineralization of water [2]. These resins consist of three-dimensional covalent networks to which bound ions are attached. While ion-exchange beads can be very effective, it should be noted that they have a number of drawbacks. During the activation stage, solvents *Corresponding author. Tel.: 11-217-356-7162; fax: 11-217352-6655. E-mail address: [email protected] (C.L. Mangun).
must be used to pre-swell the cross-linked beads to facilitate sulfonation. The beads are very susceptible to fracture and breakage due to osmotic shock and must be kept wet at all times. The beads frequently require costly service containment systems. More importantly, there are a number of major needs with respect to environmental pollution where greatly improved ion-exchange materials are required. These needs include materials with very high contact efficiencies, a capability for rapid regeneration, and a much longer service life. Activated carbons have been primarily used for the removal of molecules, by physical adsorption, from waste streams. High surface area activated carbon fibers (ACFs) were first prepared in 1969 by direct carbonization and activation of a phenolic fiber in steam / CO 2 at 800– 1000 8C, and are commercially available through Nippon Kynol姠 [3,4]. The ability to tailor the micropore size and surface chemistry of the fibers has been shown to markedly enhance adsorption of trace contaminants from air and water [5–8]. Furthermore, the fibrous nature provides excellent contact efficiency with contaminated media, resulting in rapid adsorption and desorption rates [9].
0008-6223 / 02 / $ – see front matter 2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 02 )00146-X
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These unique properties, coupled with excellent chemical and thermal resistance, provided an incentive to investigate the preparation and evaluation of high surface area carbonaceous ion exchanging fibers. Oxidation of carbons to prepare weak acid cationic (WAC) exchangers has been extensively studied [6,10– 12]. The ability to functionalize carbons with strong acid cationic (SAC) groups would lead to systems with ionexchange capability over the entire pH range. The preparation of high surface area carbons typically involves treating precursors to high temperature (700–1100 8C) in activating environments [13]. Roman-Martinez et al. [14] reported that the degradation of phenolic resins at temperatures above 700 8C was associated with a loss of aromatic hydrogen. The presence of aromatic hydrogen is essential for electrophilic substitution reactions such as sulfonation. Due to the poor reactivity of carbons activated at temperatures above 700 8C, reports of sulfonation in the literature are scarce. Instances where it has been accomplished are for coals with sulfuric acid and carbon blacks with oleum, albeit with low ion-exchange capacities ranging from 0.1 to 0.79 mEq g 21 [15,16]. The scope of this work encompasses the preparation of SAC exchange fibers derived from phenolic resins. Principal variables investigated to maximize the degree of sulfonation included the carbonization and sulfonation temperatures. In order to retain the aromatic hydrogen necessary for sulfonation, phenolic fibers were thermally and chemically activated at significantly lower temperatures than those normally used [13]. The method for preparation of the cationic exchange fibers will be covered as well as the optimization of the activation process and characterization of end products (including ion-exchange capacity at low / high pH). The process of carbonization is not complete until the heat treatment temperature exceeds 1000 8C. Below this temperature, the materials have intermediate properties between polymer and carbon and are thus termed pyropolymers or, alternatively, chars [17]. Because the approaches described involve low temperature treatments (,600 8C), considerable retention of the organic functionality is observed in the infrared spectra and elemental compositions of the fibers. Thus, it is believed that the terms pyropolymer or char are more descriptive and appropriate than carbon for these systems.
2. Experimental
2.1. Preparation of ion-exchange fiber samples 2.1.1. Activation of fibers In this work, three sets of precursors to ion-exchange fibers were prepared. The first fiber set, derived from the phenolic fiber Kynol姠, has been described previously [18]. Kynol姠 fibers were heated under 400 ml min 21 dry
nitrogen flow at a desired temperature between 500 and 600 8C to make thermally activated phenolic fibers (TAPFs). The second set of fibers was prepared following a patent by Economy et al. [19]. Preweighed samples of Crane 232 chopped glass fibers were dipped into a phenolic resin solution (25 g of Novolac resin, 1.7 g of hexamethylenetetramine, and 100 ml of ethanol) and placed in a Lindberg tube furnace under dry nitrogen at a flow rate of 400 ml min 21 . The coated fiberglass was cured by first heating to 100 8C for 20 min to remove the solvent, then heating at 10 8C min 21 to 150 8C, and finally heat treating it isothermally at 150 8C for 20 min. The resultant fiber had a 61.4 wt% resin coating. In a separate experiment, the resin concentration in ethanol was reduced (12.5 g of Novolac resin, 0.85 g of hexamethylenetetramine, and 100 ml of ethanol) to produce thinner carbon coatings supported on the fiber substrate (38.0 wt%). The composite fibers were activated at 525 8C to prepare thermally activated phenolic coatings supported on glass fiber substrates (TAPG). With the third set of fibers, a chemical activation method developed by Yue et al. [20] was employed. Preweighed samples of Crane 232 chopped glass fibers were dipped into a phenolic resin solution (15 g of Novolac resin, 1.0 g of hexamethylenetetramine, 13.95 g of ZnCl 2 , and 100 ml of ethanol) and placed in a Lindberg tube furnace under dry nitrogen at a flow rate of 200 ml min 21 . The coated fiberglass was heated to 100 8C for 20 min to remove the solvent, then cured by heating at 10 8C min 21 to 150 8C and held isothermally for 20 min. The resultant fiber had a 55.9 wt% resin coating. The resin concentration in ethanol was reduced (7.5 g of Novolac resin, 0.5 g of hexamethylenetetramine, 13.95 g of ZnCl 2 and 100 ml of ethanol) in order to produce a thinner carbon coating supported on the fiber substrate (35.5 wt%). All of the composite fibers prepared in this way were activated under nitrogen flow at 350 or 450 8C to produce an activated coating on the glass fibers. The fiber was washed with distilled water followed by 0.1 N HCl three times to remove residual zinc compounds from the accessible pore structure. These fibers will be referred to as chemically activated phenolic coatings supported on glass fiber substrates (CAPG).
2.1.2. Functionalization Approximately 1 g of each of the activated fibers was functionalized in 150-ml concentrated sulfuric acid (Fisher Scientific, 98% purity by volume) for 3.0 h at temperatures ranging between 160 and 250 8C. The acid was preheated in a glass beaker with continuous stirring. Substantial evolution of bubbles was noted upon fiber addition and definite degradation occurred as confirmed by weight loss and a solution color change from clear to amber. The samples are denoted using the fiber type, the carbonization temperature, and the sulfonation temperature (e.g. TAPF525-160 is a thermally activated phenolic fiber carbonized
K.R. Benak et al. / Carbon 40 (2002) 2323 – 2332
at 525 8C followed by sulfonation at 160 8C). After sulfonation, the samples were rinsed thoroughly with 0.1 M NaHCO 3 solution to neutralize any residual acid. The ion-exchange sites in the fibers were then regenerated with 2 N HCl, rinsed thoroughly with distilled water, and dried for 12 h in air at 130 8C prior to ion-exchange capacity determinations.
2.2. Elemental analysis ( EA) Elemental analysis was performed using a model 240XA Elemental Analyzer (Control Equipment Corporation). The nitrogen, carbon, hydrogen, and sulfur contents were determined directly while the oxygen content was calculated by difference. For brevity, nitrogen and / or zinc contents were not reported since the amount of these elements was not significant (,1%).
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aliquot of the solution was removed and titrated with standardized 0.1 M sodium hydroxide solution using a phenolphthalein indicator. The volume of sodium hydroxide used, times the normality per gram of exchange material, is representative of the normalized loading capacity in mEq g 21 units. For a combined measurement of strongly and weakly acidic groups, 100 ml of 0.05 M sodium hydroxide solution (excess) and 0.5 g of sample were added to a bottle beaker. The contents were shaken for 4 h to ensure that the solution had come to equilibrium. The solution was then back titrated with standard 0.1 M hydrochloric acid solution using a phenolphthalein indicator. The mmol of hydrochloric acid subtracted from the mmol of initial sodium hydroxide per gram of exchange material is representative of the normalized loading capacity in mEq g 21 units.
2.6. Nitrogen and carbon dioxide adsorption 2.3. Diffuse reflectance fourier transform infrared spectroscopy ( DRIFTS) A Magna IR TM Spectrometer 550 (Nicolet) was used in reflectance mode to measure the infrared spectrum of the fiber samples and to identify their chemical functionalities. Data acquisition was performed automatically using an interfaced computer and a standard software package (Omnic). The samples were dried under vacuum at 150 8C prior to mixing with KBr powder (|10 wt% carbon). The samples were run in ratio mode allowing for subtraction of a pure KBr baseline. The sample chamber in the spectrometer was purged with nitrogen (20 min) to remove carbon dioxide and water. A total of 64 scans were then taken for each sample at a resolution of 4 cm 21 using triangular apodization to smooth the noise level.
2.4. Thermal gravimetric analysis ( TGA) All thermal gravimetric analyses were obtained using a TGA 951 (TA Instruments, New Castle, Delaware) interfaced with a TA Instruments 2100 computer. The overall thermal stability of functional groups added through sulfonation was monitored by heating the samples at 10 8C min 21 to 800 8C in an inert nitrogen atmosphere. Coating contents of impregnated glass samples were determined from the mass loss of the sample due to combustion using a heating rate of 10 8C min 21 to 700 8C in air. The gas flow rate was 100 ml min 21 and the approximate weight of individual samples was 50 mg.
2.5. Ion exchange capacity measurements The cationic exchange capacity of strongly and weakly acidic groups was determined in a conventional manner [21]. For strongly acidic groups, 50 ml of 1 M sodium chloride solution (excess) and approximately 0.5 g fiber were added to a bottle beaker and stirred for 4 h. A 25-ml
A Quantachrome Autosorb-1-MP (Boynton Beach, FL) was used for the volumetric measurement of nitrogen and carbon dioxide adsorption isotherms. The adsorption temperature for nitrogen was 77 K while the adsorption temperature for carbon dioxide was 298 K. All samples were degassed for 3 h under vacuum at 130 8C. The adsorption experiments were performed in static mode using a manifold set to supply an adjustable dose of adsorbate to the sample cell. After equilibration, the pressure was recorded and the process was repeated at higher dose pressures until the saturation pressure was reached. The pressure difference between the previous pressure measurement and the next equilibrated pressure along with the dose volume was used to calculate the amount of adsorbate uptake by the sample. Micropore surface area and micropore volumes were calculated from the experimental CO 2 isotherm in the appropriate P/Po range using the Dubinin–Radushkevich (DR) equation. The total pore volume was estimated from the amount of nitrogen adsorbed at P/Po 5 0.95, assuming a liquid density of nitrogen at 77 K of 0.808 g cm 23 . The t-plot method, developed by Lippens and de Boer, was used to estimate the micropore volume from nitrogen isotherms.
3. Results and discussion
3.1. Ion-exchange fibers derived from TAPF 3.1.1. Characterization of surface chemistry of sulfonated fibers Results from elemental analysis, in conjunction with infrared spectroscopy, were used to characterize the surface chemistry of TAPFs prior to and after sulfonation. Conley et al. [22] have extensively studied the thermal degradation mechanism of phenolic resins. High temperature treatment (.500 8C) under inert atmospheres led to
K.R. Benak et al. / Carbon 40 (2002) 2323 – 2332
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Fig. 1. Infrared spectra of TAPF series before sulfonation: (a) TAPF-600, (b) TAPF-575, (c) TAPF-550, (d) TAPF-525.
the reduction of phenolic hydroxyl functionality and an extensive oxidation from within the resin to form quinone and acid fragments on the surface. Infrared and elemental analysis results collected of samples prior to sulfonation were consistent with these early reports. Infrared spectra of all TAPFs directly after carbonization are depicted in Fig. 1. Reduction of phenolic hydroxyls (3546 cm 21 ) was observed as the heat treatment temperature was increased. Absorptions at 1683 and 1709 cm 21 indicated the presence of carbonyl groups. The absorbance at 1600 cm 21 is found with all oxidized carbons and was attributed to C=C stretching of polyaromatic systems. Furthermore, the absorbance at 2914 cm 21 decreased with respect to that at 3049 cm 21 indicating a change from aliphatic CH linkages to aromatic CH bonds. Morterra et al. [23] assigned absorbances in the range of 1150–1360 cm 21 as aromatic C–O stretches and phenolic OH deformation. Other peaks including 1462, 881, 814, and 748 cm 21 can be attributed
to CH bending and deformation. From elemental analysis results listed in Table 1, H / C and O / C mole ratios decreased as carbonization temperatures increased, indicating a greater loss of hydrogen and oxygen at higher heat treatment temperatures. After sulfonation, elemental analysis and infrared spectra indicated significant changes in the surface chemistry of the fibers. Regardless of the carbonization temperature, the sulfonation process incorporated sulfur and increased the oxygen contents significantly, with greater increases observed for the higher sulfonation temperature (Table 1). The increase in oxygen was significantly higher than expected as calculated from the addition of only sulfonic acid groups. Thus, it was concluded that oxidation of the microporous carbon accompanied the high temperature sulfonation of these systems. This result is consistent with reports in the literature [24]. By examining either the 160 or 220 8C sulfonation temperature, increases in the S / C and O / C ratios were correlated with decreasing carbonization temperatures. This indicated an increase in reactivity of the structure at lower carbonization temperatures. Infrared spectra of a carbonized, sulfonated, and sulfonated sample treated with sodium hydroxide (TAPF-600 series) are shown in Fig. 2. After sulfonation, the presence of characteristic –HSO 3 absorption bands at 1034 and 605 cm 21 assigned to S=O stretching and C–S stretching, respectively, indicates incorporation of SO 3 H groups into the structure. Elemental analysis showed that significant oxidation of the structure accompanied sulfonation, an observation that was also supported by infrared spectra. Puri [25] determined that three groups (carboxylic acids, quinones, and phenolic hydroxyls) typically dominate an oxidized pore surface. In agreement with this observation, the band at 3541 cm 21 extended and broadened to incorporate lower wavenumbers. This was attributed to the formation of carboxylic acids, which in the solid phase exhibit a broad band at 3300–2500 cm 21 due to OH stretching. The absorption at 1740 cm 21 increased con-
Table 1 Elemental analysis of TAPF series Sample
Wt% Carbon
Wt% Hydrogen
Wt% Oxygen
Wt% Sulfur
H/C mol ratio
O/C mol ratio
S/C mol ratio [310 3 ]
TAPF-525 TAPF-525-160 TAPF-525-220 TAPF-550 TAPF-550-160 TAPF-550-220 TAPF-575 TAPF-575-160 TAPF-575-220 TAPF-600 TAPF-600-160 TAPF-600-220
90.0 72.8 65.0 91.2 74.5 67.7 92.4 76.0 70.4 93.8 78.5 72.1
4.29 1.81 1.99 4.18 1.78 1.96 4.06 1.74 1.92 3.95 1.71 1.89
5.72 21.8 27.5 4.6 20.6 25.8 3.50 19.4 24.2 2.38 17.9 23.0
0.0 3.62 5.57 0.0 3.17 4.5 0.0 2.78 3.49 0.0 1.92 3.04
0.57 0.30 0.37 0.55 0.29 0.35 0.53 0.27 0.33 0.51 0.26 0.31
0.05 0.22 0.32 0.04 0.21 0.29 0.03 0.19 0.26 0.02 0.17 0.24
0.0 19 32 0.0 16 25 0.0 14 19 0.0 9.2 16
K.R. Benak et al. / Carbon 40 (2002) 2323 – 2332
Fig. 2. Infrared spectra of TAPF-600-220: (a) sample prior to sulfonation, (b) H 1 form, (c) Na 1 form.
siderably and was assigned to carbonyl stretching from carboxylic acid groups. Chemical modification of the carboxylic acid groups to carboxylic acid salts further confirmed the presence of these groups in the sulfonated fibers. Sodium hydroxide solution was used to exchange the hydrogen ions of the sulfonic acid groups for sodium, resulting in a decrease in intensity of the 1740 cm 21 band and the broad band centered at 3500 cm 21 . A new band at 1385 cm 21 appeared and is due to the stretching of carboxylate groups. Fig. 3 shows infrared spectra of the entire TAPF series after sulfonation at 160 8C. The sample carbonized at 600 8C had a very noticeable decrease in the ratio of absorbance of S=O with respect to phenolic OH deformation, indicating a relative decrease in the amount of sulfonic acid units.
Fig. 3. Infrared spectra of TAPF series after sulfonation at 160 8C: (a) TAPF-600-160, (b) TAPF-575-160, (c) TAPF-550-160, (d) TAPF-525-160.
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3.1.2. Thermal stability of sulfonated fibers The thermal stability of these materials is directly dependent on the decomposition temperature of the various functional groups. For example, the sulfonic acid group (–HSO 3 ) thermally decomposes near 250 8C, while oxygen containing functionality such as carboxylic acids, quinones, and phenolic hydroxyls have a higher thermal stability [24]. It is generally accepted that CO 2 is given off as a volatile from oxidized carbon surfaces at #350 8C mainly due to the decomposition of carboxylic acid groups, whereas the loss of CO is derived from the decomposition of phenolic hydroxyls and quinones at higher temperatures [26,27]. The thermal degradations of sample TAPF-600 prior to and after sulfonation were examined and the superimposed plots of mass change as a function of temperature are presented in Fig. 4a. The weight loss as a function of temperature provides an indication of the functional groups existing prior to or incorporated by the sulfonation treatment, which are decomposed as volatiles during heat treatment. Before sulfonation, the sample lost only 13.3% while the sulfonated sample lost 59.5% of its initial mass upon heating to 800 8C (this value does not include the initial loss, attributed to strongly adsorbed water). Thus, sulfonation increased the weight loss associated with functional groups by 4.5 times. Differential thermogravimetry (DTG) was used for thermal stability analysis of functional groups on sample TAPF-600 prior to and after sulfonation (Fig. 4b). The DTG curve was used to estimate the initial and final degradation temperatures for the sample [28]. Before sulfonation, the fiber was thermally stable up to 491 8C while the sulfonated sample began to lose discernible weight at 230 8C due to sulfonic acid groups followed by additional degradation due to the various acidic functional groups as described above. Even commercially available cationic exchange beads specifically designed with high thermal stability have a maximum use temperature of only 130 8C [29]. Therefore, these systems may find use in many industrial applications including the sorption of cations from corrosive media, hot solutions or melts. 3.1.3. Ion-exchange capacity of sulfonated fibers Elemental and FTIR analysis have confirmed that the sulfonation process created sulfonic acid functionality that can effectively exchange cations at all pH values. Fig. 5 shows the capacity, due to strongly acidic sulfonic acid groups, as a function of carbonization and sulfonation temperatures. The data shows that at higher carbonization temperatures the reactivity of the fiber for sulfonation decreases, possibly due to deactivation of the ring structure. On the other hand, capacity increased significantly as sulfonation temperature increased. As discussed previously, it was found that oxidation of the microporous carbon accompanied the high temperature sulfonation of these systems, creating carboxylic acid and phenolic hydroxyl functionalities that further enhance the exchange capacity
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Fig. 5. Strong acid ion exchange capacities of TAPF series as a function of carbonization and sulfonation temperatures.
cially available beads (Purolite C100X10H) was measured to be 4.7 mEq g 21 after the added step of an overnight pre-swell before testing.
3.1.4. Effect of sulfonation treatment on pore structure This work shows a correlation between the extent of functionalization, pore volume, and surface area and serves to demonstrate how sulfonation affects the porous structure of the fibers (Table 2). No significant nitrogen adsorption at 77 K was observed for samples thermally activated at temperatures below 600 8C. Daley et al. [18] determined that the porosity in thermally activated phenolic systems, in the absence of external etching agents, is due solely to the volatilization of degradation by-products. In that instance, the structure was directly imaged using STM and ˚ with the micropores were measured to be less than 11 A, Fig. 4. (a) Thermogram depicting weight loss due to decomposition of functional groups from carbon surface for (a) TAPF-600 and (b) TAPF-600-220. (b) Differential thermogram depicting decomposition peaks for (a) TAPF-600 and (b) TAPF-600-220.
of these materials at higher pH. Measuring the total ionexchange capacity of the material and subtracting the contribution from the strong acid group exchange determined the capacity of the weakly acidic functional groups. The data shows that, as the carbonization temperature increases, the capacity due to weakly acidic functionality remains stable (Fig. 6). Oxidation, unlike sulfonation, is not a substitution reaction requiring aromatic hydrogen from the carbon; therefore, higher carbonization temperatures do not affect reactivity in the same manner. In a similar fashion to the sulfonic acid groups, the concentration of oxygen groups increased with increasing functionalization temperature. Total ion-exchange capacities as high as 9.9 mEq g 21 were measured for the sulfonated fibers. For comparison, the exchange capacity of commer-
Fig. 6. Weak acid ion exchange capacities of TAPF series as a function of carbonization and sulfonation temperatures.
K.R. Benak et al. / Carbon 40 (2002) 2323 – 2332
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Table 2 Surface area and pore volume for TAPF series Sample
Micropore volume [cm 3 g] 21 (CO 2 , 298 K)
DR surface area [m 2 g] 21 (CO 2 , 298 K)
TAPF-525 TAPF-525-160 TAPF-525-220 TAPF-550 TAPF-550-160 TAPF-550-220 TAPF-575 TAPF-575-160 TAPF-575-220 TAPF-600 a TAPF-600-160 TAPF-600-220
0.22 0.12 0.15 0.25 0.15 0.18 0.28 0.18 0.21 0.29 0.19 0.21
500 285 353 571 348 416 648 417 485 670 431 499
Properties calculated from nitrogen adsorption isotherm: micropore volume50.24 cm 3 g 21 , total pore volume50.27 cm 3 g 21 . Nitrogen adsorption is diffusion limited for all other samples. a
˚ Daley et al. the majority of micropores being less than 6 A. [18] also observed small mesopores on the surface ranging ˚ When nitrogen at 77 K is used as the in width up to 50 A. adsorbate to characterize essentially microporous solids, it may present a problem because of limited diffusion rates [30]. To overcome this problem, all samples were characterized using carbon dioxide since it can be carried out at room temperature thus facilitating its entry into the narrow micropores. After thermal activation at 600 8C, the total pore volume is quite close to the micropore volume from carbon dioxide adsorption because the above constrictions are removed and the microporosity is accessible to both adsorbates. From Table 2, sulfonation treatments decreased both the CO 2 micropore volume and surface area of the fibers, presumably due to the added amount of functional groups. Higher sulfonation temperatures resulted in a slight upswing of surface area, undoubtedly due to the additional etching of the structure that accompanies more aggressive treatment conditions. This is a similar finding to those of other investigators studying the effect of nitric acid oxidation on activated carbon pore structure [6,10]. Samples thermally activated at 600 8C that were subsequently sulfonated lost their capacity to adsorb nitrogen at 77 K. The sulfonic acid groups must add to the mesoporous regions found at pore openings, reported by Daley et al. [18], essentially blocking or significantly slowing nitrogen adsorption. Menendez et al. [31] also observed preferential
addition of functional groups to the external surfaces of carbons due to diffusional limitations of their oxidizing agents.
3.2. Ion-exchange fibers derived from TAPG Thermally activated phenolic coatings on glass were prepared using the optimized conditions determined for thermally activated phenolic fibers previously described (TAPF-525-220) in order to assess any benefits that might be associated with glass fiber supported coatings. Prior to sulfonation, the carbon coatings had identical structures and properties to those seen with thermally activated phenolic fibers. The FTIR, elemental analysis, surface area and micropore volume (normalized per gram of carbon), showed no significant differences and were excluded for brevity. Thus, the only variable of concern relates to coating thickness / carbon loading and its effect on ionexchange properties. In this study, impregnation loadings of 28.0 and 49.0 wt% (Table 3) were utilized. It was determined that high loadings produced results similar to those of the TAPFs. However, with thinner coatings, the capacity per gram of carbon increased for the strongly acidic functional groups due to the increased surface to volume ratio. This supports the notion that the sulfonic groups are primarily located near the mesopore openings of the fibers. In addition, elemental analysis confirmed
Table 3 Properties of ion-exchange fibers derived from TAPGs Sample
TAPG-525-220 TAPG-525-220
Coating (wt%)
28.0 49.0
Capacity [mEq g 21 coating] Strongly acidic
Weakly acidic
2.5 1.8
8.1 8.3
S/C mol ratio [310 3 ] 50 33
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these findings as the sulfur to carbon ratio increased for the thinner coatings. An interesting point to note is that the capacity due to weakly acidic functional groups is similar regardless of coating thickness.
Table 5 Coating percentage, surface area, and pore volume of CAPG series Sample
Coating (wt%)
Micropore volume [cm 3 g 21 coating] (CO 2 , 298 K)
DR surface area [m 2 g 21 coating] (CO 2 , 298 K)
CAPG-350 a CAPG-350-160 CAPG-350-190 CAPG-350-220 CAPG-350-250 CAPG-450 a CAPG-450-160 CAPG-450-190 CAPG-450-220 CAPG-450-250
53.4 61.8 51.4 48.0 42.0 48.2 49.0 47.6 46.8 41.9
0.44 0.18 0.22 0.28 0.31 0.59 0.46 0.50 0.57 0.64
1023 408 517 661 724 1375 1057 1170 1322 1485
3.3. Ion-exchange fibers derived from CAPG 3.3.1. Characterization of surface chemistry /pore structure Comparison of thermally and chemically activated systems shows that they have similar H / C ratios (Table 4) resulting in equivalent amounts of sulfonic acid groups; however the number of acidic functional groups is greatly increased for the CAPG. Prior to sulfonation, both chemically activated fibers (CAPG-350 / 450) have higher surface areas (versus TAPF) as calculated from CO 2 adsorption (Table 5). As discussed previously, the sulfonic acid groups are confined to the external surface of the fibers, thus leaving a greater number of sites to accommodate the weak acidic groups. Teng et al. [32] have shown that chemical activation of phenolic resins resulted in larger average pore sizes and broader pore size distributions than physical activation processes. This is supported by noting the difference between the micropore volume from CO 2 and the total pore volume from nitrogen, which indicates a shift towards mesopores. In addition, increased surface area and pore volume retention is observed after sulfonation, with more aggressive conditions leading to better retention. This suggests that the carbonaceous structure is more reactive to sulfuric acid resulting in increased etching to offset the weight gain caused by functional groups. 3.3.2. Ion-exchange properties Figs. 7 and 8 display the strong and weak ion-exchange capacities of the CAPGs. Note that the strong acid capacities of the CAPGs and the TAPGs are comparable due to their similar amounts of sulfonic groups, although sulfonation treatment at a higher temperature of 250 8C for the CAPG did achieve a better ion-exchange capacity. On the other hand, the weak acid capacity was dramatically
a
Properties calculated from nitrogen adsorption isotherm: CAPG-350 micropore volume50.36 cm 3 g 21 , total pore volume5 0.54 cm 3 g 21 , CAPG-450 micropore volume50.53 cm 3 g 21 , total pore volume50.73 cm 3 g 21 .
Fig. 7. Strong acid ion exchange capacities of CAPG series as a function of carbonization and sulfonation temperatures.
Table 4 Elemental analysis of CAPG series Sample
Wt% Carbon
Wt% Hydrogen
Wt% Oxygen
Wt% Sulfur
H/C mol ratio
O/C mol ratio
S/C mol ratio [310 3 ]
CAPG-350 CAPG-350-160 CAPG-350-190 CAPG-350-220 CAPG-350-250 CAPG-450 CAPG-450-160 CAPG-450-190 CAPG-450-220 CAPG-450-250
90.6 66.4 57.1 45.0 38.1 94.2 82.3 73.5 61.5 49.4
3.75 2.18 2.47 2.85 3.01 3.35 1.58 1.88 2.29 2.71
5.64 29.2 36.9 47.0 52.2 2.42 14.7 22.4 32.7 43.4
0.0 2.27 3.52 5.12 6.72 0.0 1.44 2.27 3.52 4.48
0.50 0.39 0.52 0.76 0.95 0.43 0.23 0.31 0.45 0.66
0.05 0.33 0.48 0.78 1.03 0.02 0.13 0.23 0.40 0.66
0.0 13 23 43 66 0.0 6.6 12 21 34
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fibers and the application of these systems for heavy metal removal from contaminated liquids.
Acknowledgements We would like to thank NSF (Grant [ DMR 97-12489) for supporting this research. We would also like to acknowledge and thank Joe Hayes of American Kynol Inc. for his continuing support of this program and for providing the Kynol fabric, Jan Nimrick and Rudiger Laufhutte in the Microanalysis Laboratory for elemental analysis results, and Crane & Company for kindly providing glass fiber substrates. Purolite provided the ion-exchange beads.
Fig. 8. Weak acid ion exchange capacities of CAPG series as a function of carbonization and sulfonation temperatures.
increased due to the incorporation of more functional groups into the porous structure. Total ion-exchange capacities as high as 17 mEq g 21 were measured, these being much greater than any values reported in the literature. In a similar fashion to the TAPGs, strongly acidic ion-exchange capacities were enhanced through thinner coatings due to the increased surface to volume ratios of the fibers (CAPG-350-250, 29.9% coating, 2.8 mEq g 21 ). This further confirms that the sulfonic acid groups are concentrating at the surface of the fibers, even with chemically activated samples of higher initial surface area.
4. Conclusions Strong acid and weak acid cationic exchange groups were incorporated into thermally and chemically activated phenolic fibers during sulfonation. The amount and type of functional groups can be varied by careful control of the precursor fiber, carbonization temperature, and sulfonation temperature. The ion-exchange capacity of the fibrous systems is competitive with those of cationic exchange beads at low pH and substantially better with higher pH influents. This characteristic can be very useful in industrial practice, since hard water is naturally buffered at pH .7. Material durability is equally important as capacity in ion-exchange processes. Use of carbonaceous ionexchangers eliminates several problems encountered with ion-exchange beads. For example, the fibers will not swell and therefore will not undergo osmotic shock. Secondly, the fibers are more thermally stable and do not lose significant weight at temperatures below 250 8C, which is the degradation temperature of sulfonic acid groups. Future studies will compare reaction rates of the beads versus
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