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Ethanol removal using activated carbon: Effect of porous structure and surface chemistry
Microporous and Mesoporous Materials 120 (2009) 62–68
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Ethanol removal using activated carbon: Effect of porous structure and surface chemistry A. Silvestre-Albero, J. Silvestre-Albero *, A. Sepúlveda-Escribano, F. Rodríguez-Reinoso Laboratorio de Materiales Avanzados, Departamento de Química Inorgánica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain
a r t i c l e
i n f o
Article history: Received 21 May 2008 Received in revised form 2 September 2008 Accepted 17 October 2008 Available online 26 October 2008 Keywords: Activated carbon Ethanol removal Surface chemistry Porous structure VOCs
a b s t r a c t Activated carbons with increasing porosity have been prepared by chemical activation of olive stones using ZnCl2 followed by physical activation with CO2. The development of porosity and surface area with burn-off favours the adsorption capacity for ethanol. However, the total amount adsorbed (g/100 g AC) achieves a maximum for the sample with 30% burn-off, this amount decreasing thereafter. Apparently, for a low boiling point alcohol such as ethanol there is a critical pore size which allows an optimum packing of the adsorbed ethanol molecules. A further broadening of the porosity becomes detrimental due to the decreased overlapping adsorption potential inside the micropores. Incorporation of surface functionalities (oxygen surface groups) on the activated carbon enhances the adsorption capacity through the development of specific interactions between the ethanol molecule and the oxygen surface groups. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Environmental regulations are forcing the chemical industry to reduce the emissions of volatile organic compounds (VOCs) both in gas and liquid streams [1]. Among the different VOCs, ethanol is considered one of the most abundant. Ethanol was traditionally originated from bakeries, distilleries and foundry plants. However, the recent incorporation of ethanol in gasoline to replace high octane compounds (aromatics) has increased the emissions of unburned ethanol to the atmosphere. Although ethanol is not considered an extremely harmful pollutant, at high concentrations (above 1000 ppm) ethanol can cause severe health problems such as eye, skin and respiratory tract irritation together with negative environmental effects [2]. Removal of VOCs (e.g. ethanol) can be achieved using different approaches: bio-filtration, adsorption, catalytic combustion, etc. [3–7]. Among the different technologies, bio-filtration is probably one of the most widely applied. Bio-filtration is based on the conversation (biodegradation) of the VOC pollutant into a non-toxic compound using micro-organisms immobilized on the surface of a packing bed. Unfortunately, these bio-systems exhibit important drawbacks such as lack of accurate control of the pH, poor adaptability to changes in gas composition, etc. Adsorption has been proposed as an excellent alternative for the removal of volatile organic compounds avoiding the aforementioned drawbacks [8–13]. At this point, it is noteworthy to * Corresponding author. Tel.: +34 965 90 93 50; fax: +34 965 90 34 54. E-mail address: [email protected] (J. Silvestre-Albero). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.10.012
mention that ethanol adsorption is of paramount importance not only for pollution control but also in the production of bio-ethanol from biomass [14]. In fact, ethanol must be completely recovered from the fermentation broth due to its inhibitory character for certain micro-organism used in the fermentation process. Among the different adsorbents, activated carbons are probably the most interesting candidates, not only because of their low cost, but also because of their tuneable textural and chemical properties [15]. Previous studies concerning the removal of volatile organic compounds using activated carbons have shown that both the porous structure and surface chemistry exhibit an important role defining the adsorption properties. Additionally, these effects have been shown to be highly sensitive to the nature of the adsorptive molecule [8– 13]. Unfortunately, many of these studies do not present the exhaustive textural and chemical characterization required to define general trends or they use a wide variety of samples coming from different origins (different inorganic matter content, etc.) which does not facilitate a direct comparison between them. With this in mind, the goal of this work is to study the adsorption of a common volatile organic compound such as ethanol using a series of activated carbons prepared from the same lignocellulosic precursor (olive stones). The effect of the porous structure on the adsorption properties has been analyzed using a series of carbons with increasing burn-off while the effect of surface chemistry (oxygen surface groups) has been studied using post-synthesis modifications, i.e. an oxidation treatment with HNO3 followed by the selective removal of some oxygen surface groups.
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2. Experimental section A series of activated carbons were prepared by combining chemical and physical activation of olive stones as a precursor. Initially, the lignocellulosic precursor (particle size between 1.7 and 2.0 mm) was impregnated with an aqueous solution of ZnCl2 at 358 K during 7 h under stirring. After the impregnation, the sample temperature was increased up to 373 K in order to evaporate part (70%) of the remaining solution, the final sample being filtered and dried overnight at 353 K. The impregnated sample was submitted to a heat treatment (2 K/min) under a flow of N2 (100 cm3/min) at 773 K for 3 h. After cooling down to room temperature, the carbonized sample was washed with diluted HCl acid (10%) until complete removal of ZnCl2, followed by washing with distilled water until pH 7. Finally, the clean sample was carbonized under a N2 flow (100 cm3/min) at 1123 K for 2 h. The as-synthesized sample was labelled AC0. In a second step, the sample AC0 was submitted to physical activation with CO2 (100 cm3/min) at 1098 K using different periods of time, i.e. 20, 40, 60, 72 and 80 h, which corresponds to a burn-off of 22, 30, 48, 58 and 70%, respectively. The samples were labelled AC followed by the activation time used in each case. In order to study the effect of surface functional groups in the adsorption process, sample AC40 was submitted in a subsequent step to an oxidation treatment with HNO3 (6 M) during 1 h at boiling temperature. The oxidized sample (AC40Ox) was washed with distilled water and subsequently heat treated under He (100 cm3/ min) at 673 K and 973 K in order to selectively remove some of the oxygen surface groups. N2 and CO2 adsorption–desorption isotherms were obtained in a home-made fully automated equipment at 77 K and 273 K, respectively. Before the experiment, the samples were outgassed at 423 K for 4 h under vacuum (10 3 Pa). The ‘‘apparent” surface area was obtained using the BET method. The micropore volume (Vo) was deduced from the N2 adsorption data using the Dubinin–Raduskevitch (DR) equation, while the mesopore volume (Vmeso) was obtained as the difference between the total pore volume (Vt) adsorbed at p/po 0.95 and the micropore volume (Vo). The pore volume corresponding to the narrow microporosity (Vn) was obtained after application of the DR equation to the CO2 adsorption data [16]. Temperature-programmed desorption experiments under helium were carried out to evaluate the amount and nature of the oxygen surface groups present in each sample. Thus, 100 mg of sample were placed in a U-shaped quartz reactor on-line coupled to a quadrupole mass spectrometer (Balzer MSC200). The experiments were performed up to 1273 K under a He flow (50 ml/ min) and a heating rate of 10 K/min. The amount of CO and CO2 evolved were quantified after calibration with calcium oxalate (CaC2O4 H2O). Immersion calorimetry measurements into ethanol (Panreac, ethanol absolute partially denatured QP) and ultrapure water were performed in a Tian-Calvet C80D calorimeter at 303 K. A complete description of the experimental set-up can be found elsewhere [17]. Briefly, the sample was outgassed at 423 K for 4 h in a glass tube connected to a vacuum equipment. After the heat treatment the bulb containing the sample was sealed in vacuum and then it was introduced into the calorimetric cell containing the immersion liquid. Once the thermal equilibrium was reached, the glass bulb tip was broken and the wetting liquid was allowed to contact the sample. The heat evolved as a result of this interaction was recorded as a function of time. The integration of the signal, after appropriate corrections (the breaking of the tip (exothermic) and the heat of evaporation of the immersion liquid to fill the empty volume of the bulb with the vapour at the corresponding vapour
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pressure (endothermic), both contributions being previously calibrated using empty glass bulbs with different volumes), provides the total enthalpy of immersion ( DHimm). Areal enthalpy of immersion (mJ/m2) was calculated from the enthalpy of immersion (J/g) using the ‘‘apparent” BET surface area obtained from the N2 adsorption isotherms at 77 K. Ethanol breakthrough column experiments were performed in a U-shaped reactor connected on-line to a gas-chromatograph (Agilent GC 6890N). In a common experiment, 500 mg of AC were heat treated in clean air (1000 cm3/g) at 423 K and 523 K, in order to remove water humidity. Then, the sample was cooled down to the adsorption temperature (298 K) and it was contacted with an air flow (1000 cm3/min) containing a concentration of 250 ppmv of ethanol (saturation pressure for ethanol at 265.8 K is 93 Pa). After saturation, the weakly adsorbed ethanol was removed and analyzed by passing an air flow (1000 cm3/min) at 298 K, while the strongly adsorbed ethanol was desorbed and quantified by temperature-programmed desorption analysis (up to 573 K). 3. Results and discussion 3.1. Sample characterization 3.1.1. N2 adsorption–desorption isotherms N2 adsorption–desorption isotherms at 77 K for the different activated carbons are shown in Fig. 1a. Additionally, Table 1 reports the textural characteristics obtained from the N2 and CO2
Fig. 1. N2 adsorption–desorption isotherms at 77 K for (a) activated carbons with increasing burn-off (0, 20, 40, 60, 72 and 80 h) and (b) activated carbon AC40 modified by an oxidation treatment with HNO3 (6 M) followed by a thermal treatment at low (473 K) and high temperature (973 K).
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Table 1 Specific surface area (SBET) and total pore volume (Vt) calculated from the N2 and CO2 adsorption isotherms at 77 K and 273 K, respectively, for the different activated carbons. Sample
N2 adsorption
CO2 adsorption
SBET (m2/g)
Vo (cm3/g)
Vmeso (cm3/g)
Vt (cm3/g)
Vo/Vt
Vn (cm3/g)
AC0 AC20 AC40 AC60 AC72 AC80
866 1253 1448 1983 2097 2127
0.33 0.46 0.54 0.70 0.74 0.76
0.04 0.07 0.09 0.23 0.26 0.26
0.37 0.53 0.63 0.93 1.00 1.02
0.89 0.87 0.86 0.75 0.74 0.74
0.26 0.36 0.40 0.45 0.46 0.47
AC40Ox AC40Ox/HT673 AC40Ox/HT973
1264 1319 1444
0.48 0.49 0.53
0.11 0.11 0.12
0.59 0.60 0.65
0.81 0.82 0.82
0.40 0.38 0.38
adsorption isotherms at 77 K and 273 K, respectively. As it can be observed, all carbons exhibit a Type I isotherm according to the IUPAC classification, characteristic of microporous materials [18]. The as-synthesized chemically activated carbon (AC0) exhibits a narrow knee at low relative pressures, suggesting the presence of slightly narrow micropores. This observation is confirmed by comparing the relatively similar values of micropore volume obtained from the N2 and CO2 adsorption isotherms (see Table 1) [16]. It is noteworthy to mention that application of the DR equation to the N2 adsorption isotherms provides the total volume of micropores (those below 2 nm) while the same equation applied to the CO2 adsorption isotherms provides the volume of narrow micropores (those below 0.6–0.7 nm). Thus, in the absence of kinetic restrictions these two values (Vo, from N2 at 77 K, and Vn, from CO2 at 273 K) must be similar for carbons exhibiting narrow and homogeneous micropores, while they become different (Vo > Vn) with the broadening of the porosity. The subsequent activation of AC0 with CO2 gives rise to an increase in the total amount adsorbed, i.e. an increase in the micropore volume, together with a broadening of the knee in the N2 isotherm. Thus, the activation treatment produces both the opening of new micropores together with the broadening and deepening of the existing ones. The broadening of the microporosity is clearly reflected both comparing the difference between Vo and Vn, and also considering the variation of the relative ratio Vo/Vt (percentage of microporosity) with burn-off. In this sense, the percentage of microporosity slightly decreases with the activation degree down to sample AC40, this value decreasing faster to a relatively common value afterwards (together with a noticeable increase in the mesoporous volume). ‘‘Apparent” BET surface area range from 866 m2/g on the chemically activated carbon (AC0) to 2127 m2/g on the sample with 70% burn-off (AC80). Fig. 1b shows the N2 adsorption–desorption isotherms at 77 K for the activated carbon AC40 before and after the oxidation treatment with HNO3, together with the isotherms after the heat treatment in He at 673 K (AC40Ox/HT673) and 973 K (AC40Ox/HT973). The oxidation treatment with a strong oxidizer such as HNO3 produces some changes in the adsorption properties of sample AC40, although its microporous nature is preserved. The oxidized sample (AC40Ox) exhibits a decrease in the total adsorption capacity, in the surface area and in the total pore volume (see Table 1), with no change in the volume of narrow microporosity. Previous studies on activated carbons have shown that the effect of an oxidation treatment with HNO3 highly depends on the nature of the carbon sample, the concentration of nitric acid and the extent (time) of the oxidation process [19–21]. In this sense, the oxidation treatment can modify only the chemical nature of the carbon surface with mainly no effect in the porosity or it can produce the collapse of small pores, thus modifying the porous structure.
A subsequent heat treatment in helium at low temperature (673 K) has mainly no effect on the adsorption properties while, after a high temperature treatment (973 K), the N2 isotherm resembles that of the original sample. These results can be explained by considering the blocking effect of the oxygen surface groups in the oxidized carbon (AC40Ox) which limit the accessibility of the nitrogen molecule to the narrow micropores at the low adsorption temperature (77 K) [19]. A heat treatment at low temperature (673 K), which only removes the most acidic and less stable oxygen surface groups, produces almost no effect on the adsorption isotherm, confirming that these groups are not responsible for the aforementioned restricted diffusion effect. However, a heat treatment at a higher temperature (973 K) causes the removal of an important part of the oxygen surface groups (the less stable groups, evolved as CO2 in temperature-programmed experiments, together with part of the more stable and less acidic oxygen groups, which evolve as CO), and their corresponding carbon atoms (selective gasification) giving rise to a slight increase in the adsorption capacity (mainly mesoporosity) in respect to the original activated carbon, AC40. In summary, these results show that the oxidation treatment of sample AC40 with a strong oxidizer (HNO3) modifies the surface chemistry with mainly no effect on the porosity. The newly created oxygen surface groups located in the pore mouth of the micropores gives rise to diffusional restrictions for the N2 molecule at 77 K. However, a subsequent heat treatment at high temperature (973 K) with He is able to completely recover the adsorption properties and surface area of the original activated carbon. 3.1.2. Temperature-programmed desorption experiments The amount and nature of the oxygen surface groups on the different activated carbons has been analyzed by temperature-programmed desorption experiments. In this sense, Fig. 2 shows the (a) CO2 and (b) CO desorption profiles for the different activated carbons in the 298–1273 K temperature range. Additionally, Table 2 reports the total amount of oxygen surface groups evolved both as CO2 and CO. Although the TPD profile for CO2 is quite complex in all cases, the contribution of the more acidic groups is quite small as compared with the amount of CO evolved (see Table 2). According to Fig. 2a, an increase in burn-off has mainly no effect in the total amount of oxygen surface groups evolved as CO2 (carboxylic acid, lactone, etc.), i.e. the least stable groups. A similar situation occurs for the oxygen surface groups evolved as CO, i.e. the most stable groups. However, in this case the amount of CO evolved exhibits a slight increase after the activation treatment during 20 h, this amount decreasing afterwards. This effect is clearly reflected in the CO/CO2 ratio, which increases continuously with the degree of activation. However, this value is not meaningful at high burn-off due to the very low amount of CO2. Thus, the activation treatment with CO2 at 1098 K slightly favours the development of oxygen surface groups, mainly those evolved as CO, although the total amount is quite small to expect any effect on the adsorption properties. The oxidation treatment with HNO3 (6 M) produces important changes in the surface chemistry. As it can be observed in Table 2, the oxidized sample (AC40Ox) exhibits an important increase in the total amount of oxygen surface groups evolved both as CO2 and CO, this enhancement being higher for the CO2 groups (lower CO/CO2 ratio), in accordance with previous FTIR results [22]. The subsequent thermal treatment at low temperature (673 K) only produces the selective removal of the more acidic and less stable oxygen groups (mainly carboxylic groups), which is clearly reflected by the increase in the CO/CO2 ratio. A further treatment at 973 K produces the suppression of the less stable groups (evolved as CO2) together with an important decrease in the amount of CO.
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Fig. 2. (a) CO2 and (b) CO temperature-programmed desorption profiles for activated carbons (a) AC0, (b) AC20, (c) AC40, (d) AC60, (e) AC72 and (f) AC80.
Table 2 Amount of CO2 and CO evolved during the TPD experiments on the as-synthesized, oxidized and heat treated activated carbons. Sample
CO2
CO
(mmol/g)
(mmol/g)
CO/CO2
AC0 AC20 AC40 AC60 AC72 AC80
0.06 0.08 0.04 0.03 0.01 0.02
0.16 0.38 0.35 0.33 0.16 0.17
2.7 4.7 8.7 11 16 8.5
AC40Ox AC40Ox/HT673 AC40Ox/HT973
1.35 0.33 0.05
2.45 2.58 0.72
1.8 7.8 14.4
3.1.3. Immersion calorimetry measurements The heat of immersion of a porous solid into a certain liquid can be used to evaluate both the porous structure and the surface chemistry of the material. In the absence of specific interactions at the solid/liquid interface, the enthalpy of immersion can be regarded as an indirect measurement of the surface area available to a certain molecule [23,24]. Thus, the appropriate selection of the immersion liquid (molecular dimensions) can be used to evaluate the surface area available to each molecule, i.e. the pore size distribution [24,25]. However, the heat of immersion for certain molecules can be altered due to the presence of specific interactions between the solid surface and the immersion liquid [26,27]. In this case, the heat of immersion can be used to evaluate the de-
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gree and the nature of the interactions at the solid/liquid interface, information that can be very useful to predict the usefulness of the solid surface into a certain application, e.g. adsorption process. In this sense, Table 3 reports the enthalpy of immersion into ultrapure water and ethanol at 303 K for the different activated carbons as-synthesized, oxidized and heat treated in He at different temperatures (673 K and 973 K). Interestingly, for all samples the enthalpy of immersion (J/g) into ethanol is higher than that obtained using a molecule with a larger dipole moment such as water (dipole moment 1.69 D for ethanol vs. 1.85 D for water), in accordance with previous observations [20,26,28]. The development of porosity, i.e. an increase in the activation degree, gives rise to an increase in the enthalpy of immersion on a unit weight basis for both liquids. Unfortunately, this tendency, which is affected by the increased surface area available to both liquids with the degree of activation, does not permit a direct comparison between the different samples. In this sense, the areal enthalpy of immersion, i.e. the heat of immersion per m2, avoids the aforementioned drawback and allows the comparison of surfaces with different porous structure, i.e. with a different development of porosity. As expected, the areal enthalpy of immersion is quite similar for all activated carbons. Thus, the degree of interaction per m2 for water and ethanol is scarcely affected by the development of the porosity. Only samples above 30% burn-off (AC40) exhibit a slight decrease, this effect being attributed either to the broadening of the porosity (the heat of immersion is enhanced in narrow micropores [29]) or to the lower amount of oxygen surface groups on these samples. The incorporation of oxygen surface groups through an oxidation treatment with HNO3 produces an increase in the enthalpy of immersion for both liquids, this effect being more important for water [19,20,26]. In fact, the ratio DHimm (H2O)/ DHimm (CH3CH2OH) changes from 0.27, in the as-synthesized AC40 sample, to 0.76, in the oxidized carbon (AC40Ox). Similar values have been described in the literature for the interaction of a similar alcohol such as methanol (dipole moment of 1.69 D) and water with the surface of activated carbons and charcoals [26,30]. The large enthalpy of immersion obtained for alcohols (methanol and ethanol) in un-oxidized activated carbons together with the small increase observed after an oxidation treatment clearly suggest that the heat of interaction between both phases, i.e. carbon material/alcohol, must be defined by the carbon surface/ alcohol interactions (dispersion interactions mainly through the hydrocarbon chain), while the oxygen surface groups/alcohol interactions must be less important. The contrary occurs for a polar molecule such as water. In this case, the un-oxidized carbons exhibit a low degree of interaction with water, in accordance with its hydrophobic character. However, the incorporation of surface functionalities (oxygen surface groups) highly enhances the hydrophilic character of the carbon, thus increasing the interaction with water and, consequently, the enthalpy of immersion [19]. Additionally, these primary adsorption sites for water on the carbon surface can act as secondary sites for further adsorption via hydrogen bonding, which in fact will enhance the adsorption enthalpy. This explains the large increase in the DHimm (H2O)/ DHimm (CH3CH2OH) ratio after the oxidation treatment. Calorimetric studies on carbon blacks using a non-polar molecule such as n-heptane and a polar molecule such as water had confirmed the larger degree of interaction with the carbon surface of the hydrocarbon chain in n-heptane compared to water [29]. A subsequent thermal treatment at low (673 K) and high temperature (973 K) gives rise to a progressive decay in the areal enthalpy of immersion, this effect being more evident for water. This behaviour indicates that the degree of interaction between a polar molecule such as water and the carbon surface is highly affected by the total amount of oxygen surface groups, independently of their nature. In fact, a close inspection of the results
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moisture impurities. It is noteworthy to mention that the temperature of this treatment constitutes a critical step defining the adsorption behaviour of activated carbons. In fact, temperatures above 523 K promote the chemisorption of oxygen from the air flow (e.g. 5 wt.% increase at 523 K), thus modifying the surface chemistry and, consequently, the adsorption–desorption properties. To avoid problem with chemisorbed oxygen, a temperature of 423 K was selected for the pre-treatment. Table 4 shows the total amount of ethanol adsorbed together with the amount weakly adsorbed (desorbed at the adsorption temperature) and strongly adsorbed (desorbed at higher temperatures). Interestingly, the amount of ethanol adsorbed (g/100 g AC) increases with the degree of activation up to a maximum on sample AC40, the total amount adsorbed decreasing thereafter. The activation treatment with CO2 gives rise to the development of porosity through the opening of new micropores together with the deepening and enlargement of the existing ones. These effects produce the creation of surface area (see Table 1) which in fact defines the total number of sites available for the adsorption of ethanol. In this sense, Fig. 3 shows the total amount of ethanol adsorbed recalculated per unit surface area available on the activated carbons with different burn-off. The adsorption capacity for ethanol is a decreasing function of the extent of activation, this showing that the microporosity, and more specifically the micropore size and not the total surface area and total pore volume, is the real factor defining the adsorption capacity for ethanol. The decrease in the amount adsorbed per unit area with burn-off should be attributed either to an increase in the diffusion pathway to reach the inner microporosity with the activation degree or, more probably, to the widening of the existing micropores which produce a reduction in the overlapping adsorption potential. Previous studies described in the literature have shown that the effect of the activation degree on the adsorption capacity per unit area of
Table 3 Enthalpy of immersion ( DHimm) into ultrapure water and ethanol for the different activated carbons. Values are referred per unit mass and per unit area, using the ‘‘apparent” surface area (SBET) calculated from the N2 isotherms. Sample
H2O
DHimm (J/g) AC0 AC20 AC40 AC60 AC72 AC80 AC40Ox AC40Ox/HT673 AC40Ox/HT973
CH3CH2OH
DHimm (mJ/m2)
DHimm (J/g)
DHimm (mJ/m2)
18.6 33.1 38.8 43.8 43.8 45.7
22.3 26.5 27.7 22.5 22.0 21.7
89.4 126.5 141.9 171.9 175.5 185.2
107.3 101.2 101.3 88.4 88.2 87.9
127.7 88.9 40.2
97.9 67.3 27.8
168.0 148.4 148.2
132.9 112.5 102.6
reported in Tables 2 and 3 show that the areal enthalpy of immersion into water (DHi) exhibits an approximately linear correlation with the total amount of oxygen surface groups [27,29]. In addition, the intercept of the straight line gives rise to an areal enthalpy of immersion for a ‘‘clean” surface (free of oxygen surface groups) of only DHi = 9 mJ/m2, thus confirming that the specific interactions between the water molecules and the oxygen surface groups are the main component defining the heat of immersion (the interaction with basic groups containing no oxygen, the filling of the micropores and the adsorption on the external surface area being negligible). In the case of ethanol, experimental results denote a better correlation between the areal enthalpy of immersion and the amount of the more acidic and less stable oxygen surface groups (evolved as CO2), although with a larger intercept value (DHi = 103 mJ/m2) [26]. The larger intercept obtained with ethanol highlights the presence of additional highly energetic specific and non-specific interactions with the carbon surface free of oxygen surface groups in the case of alcohols (mainly through the hydrocarbon tail). In this sense, an average immersion enthalpy value of DHi = 107 mJ/m2 was proposed for the dispersion interactions of a hydrocarbon molecule such as n-heptane with the surface of carbon blacks [29]. It is noteworthy to mention that the aforementioned intercept values should be considered as a rough estimation due to the limited number of experimental data available. 3.2. Ethanol adsorption–desorption experiments 3.2.1. Effect of porous structure In a common breakthrough column experiment the activated carbon is contacted with the air flow containing ethanol ([ethanol]0: 250 ppm) until complete saturation. Once the saturation has been achieved, weakly adsorbed ethanol is released by flowing clean air; the remaining more strongly adsorbed ethanol being desorbed by a subsequent temperature-programmed heating. Before the breakthrough column experiment, all samples have been treated in an air flow (1000 cm3/min) in order to remove
Fig. 3. Total amount of ethanol adsorbed per unit surface area (lmol/m2) with burn-off.
Table 4 Amount of ethanol adsorbed (g/100 g AC) at 298 K together with the amount desorbed at 298 K and that desorbed after a heat treatment up to 423 K. Sample
Ethanol adsorbed (g/100 g)
Ethanol desorbed at 298 K (g/100 g)
Ethanol desorbed at 298–423 K
% Weakly adsorbed
% Strongly adsorbed
Ethanol densitya (g/cm3)
AC0 AC20 AC40 AC60 AC72 AC80
5.49 7.13 7.37 6.20 5.57 4.78
5.45 7.12 7.28 6.13 5.54 4.75
0.03 0.00 0.09 0.07 0.04 0.03
99.2 99.9 98.8 98.9 99.3 99.3
0.5 0.1 1.2 1.1 0.7 0.6
0.21 0.20 0.19 0.14 0.12 0.10
a
Amount of ethanol adsorbed per unit volume of narrow micropores (g/cm3).
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carbon materials highly depends on the nature of the adsorptive molecule [8,11,12]. In general, for highly volatile molecules (e.g. acetone, methane, ethane, etc.) the amount adsorbed increases with decreasing pore size while for high boiling point molecules (e.g. pentane, n-hexane, etc.), the amount adsorbed increases with the development of porosity and surface area. Interestingly, the results from Fig. 3 show two distinct behaviours above and below a 30% burn-off. For samples with a low activation degree, the adsorption capacity decreases in a moderate way while for samples above 30% the decrease is slightly more drastic. To further explore the effect of the porous structure in the adsorption of ethanol, Fig. 4 represents the amount of ethanol adsorbed (mmol/g) as a function of (a) the total micropore volume, obtained from the N2 adsorption isotherms (77 K), and (b) the total volume of narrow micropores, obtained from the CO2 adsorption isotherms (273 K). There is in both cases an increase in the amount adsorbed with the pore volume up to an optimum volume around 0.55 cm3/g, for N2, and 0.40 cm3/g, for CO2. This behaviour differs considerable from that observed on larger molecules (benzene and toluene) where a continuous increase in the amount adsorbed with the total micropore volume (Vo) and total volume of narrow micropores (Vn) was observed [9]. Thus, ethanol exhibits an intermediate behaviour between that exhibited by highly volatile molecules (e.g. methane) and that exhibited by larger molecules (e.g.
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benzene). As it can be observed in Fig. 5, for activated carbons with low burn-off the development of porosity is mainly associated with the opening of new micropores together with the deepening of the exiting ones (see also Table 1), thus giving rise to an increased adsorption capacity for ethanol. However, an activation degree above 30% mainly produces the widening of the existing micropores (compare the micropore volume deduced from N2 and CO2 in Table 1 and Fig. 5), together with the development of some mesoporosity, this effect being detrimental for the adsorption of ethanol. Thus, the decreased adsorption capacity above 30% burn-off (sample AC40) must be explained due to the decreased overlapping adsorption potential in larger micropores. This statement is corroborated by comparing the total amount of ethanol adsorbed per unit volume of narrow micropores (g/cm3), which is an indirect measurement of the packing density of the adsorbed phase (see Table 4). These values show that up to 30% burn-off the density of adsorbed ethanol is quite similar and close to the value of 0.20– 0.21 g/cm3, typically present on chemically activated carbons with a narrow micropore size distribution. However, a further development of the porosity, i.e. a widening of the microporosity above a critical pore size, becomes detrimental for the packing density and, consequently, for the adsorption capacity. Interestingly, the absence of a correlation between the amount adsorbed (mmol/g) and the volume of narrow micropores (Vn) (see Fig. 4b) clearly highlights that the critical micropore size for ethanol adsorption must be slightly below the one for CO2. It is noteworthy to mention that on activated carbon materials, mainly all the ethanol adsorbed (>98%) can be easily removed at room temperature (298 K), thus favouring its regeneration. In summary, breakthrough column experiments show that ethanol adsorption at 298 K on activated carbons is highly sensitive both to the volume of narrow micropores (Vn) and to the micropore size distribution. Thus, the development of porosity favours the adsorption capacity for ethanol up to an optimum porous structure which combines a narrow micropore size distribution (high packing density) together with a highly developed narrow microporosity (Vn). A further broadening of the microporosity above the critical micropore size becomes detrimental for the adsorption process due to the decreased packing density of the adsorbed ethanol molecules, i.e. decreased overlapping adsorption potential in larger micropores. 3.2.2. Effect of surface chemistry As expected, the incorporation of oxygen functionalities on the carbon surface after an oxidation treatment highly enhances the adsorption of a polar molecule such as ethanol (see Table 5) [19].
Fig. 4. Total amount of ethanol adsorbed (mmol/g) as a function of (a) total micropore volume (Vo, obtained from the N2 adsorption isotherms at 77 K) and (b) volume of narrow micropores (Vn, obtained from the CO2 adsorption isotherms at 273 K).
Fig. 5. Evolution of the micropore volume (V0), mesopores volume (Vmeso), volume of narrow micropores (Vn) and volume of ethanol adsorbed with burn-off.
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Table 5 Amount of ethanol adsorbed (g/100 g AC) at 298 K together with the amount desorbed at 298 K and that desorbed after a heat treatment up to 423 K, for sample AC40 assynthesized and modified by oxidation and thermal treatments. Sample
Ethanol adsorbed (g/100 g)
Ethanol desorbed at 298 K (g/100 g)
Ethanol desorbed at 298–423 K (g/100 g)
% Weakly adsorbed
% Strongly adsorbed
Ethanol densitya (g/cm3)
AC40 AC40Ox AC40Ox/HT673 AC40Ox/HT973
7.37 9.76 8.15 7.07
7.28 8.96 7.94 6.99
0.09 0.79 0.21 0.08
98.8 91.8 97.4 98.8
1.2 8.1 2.6 1.2
0.19 0.25 0.21 0.19
a
Amount of ethanol adsorbed per unit volume of narrow micropores (g/cm3).
The additional carbon/ethanol interactions through the newly created oxygen surface groups favour the packing of adsorbed ethanol molecules (density of ethanol 0.25 g/cm3) on narrow micropores. However, these specific interactions inhibit the complete desorption (regeneration) of ethanol at room temperature (91% recovery). The selective removal of the less stable and more acidic oxygen surface groups during the thermal treatment in He at 673 K gives rise to an important decrease in the total amount of ethanol adsorbed. Interestingly, the removal of these groups favours the desorption of ethanol at room temperature (>97% recovery). A subsequent treatment at 973 K exhibits a similar effect with a total adsorption capacity very similar to that exhibited by the original sample (AC40). In summary, the presence of oxygen surface groups on activated carbons favours the adsorption of ethanol. However, the presence of specific interactions between the ethanol molecule and the newly created surface functionalities becomes detrimental for the subsequent regeneration of the carbon adsorbent at room temperature. Selective removal of the oxygen surface groups shows that: (i) the adsorption capacity is defined by the total amount and not by the nature of the oxygen surface groups, i.e. the amount of ethanol adsorbed decreases gradually with the total amount of oxygen surface groups, and (ii) the regeneration process is more sensitive to the nature of the oxygen surface groups, in accordance with the calorimetric data (ethanol interacts more strongly with the more acidic and less stable oxygen surface groups). 4. Conclusions Ethanol removal at 298 K has been studied using a series of activated carbons with increasing burn-off. Breakthrough column experiments show that the amount of ethanol adsorbed (g/100 g) increases with the activation degree up to a maximum on sample AC40 (7.4 g/100 g AC), the amount adsorbed decreasing thereafter. Apparently, the adsorption of ethanol on activated carbons exhibits a critical micropore size which favours an optimum packing of adsorbed ethanol molecules, i.e. probably a pore size able to accommodate two adsorbed layers of ethanol. Interestingly, activated carbons can be easily regenerated by passing an air flow at room temperature (more than 98% desorption/recovery). Incorporation of oxygen surface groups on the carbon structure enhances the adsorption capacity for a polar molecule such as ethanol. Selective removal of these surface functionalities highlights that all oxygen surface groups (those evolved as CO2 and CO in TPD experiments) participate in the adsorption process.
Acknowledgments Financial support from MEC (Projects NAN2004-09267-C03-03 and MAT2007-61734) and the Network of Excellence Insidepores (NMP3-CT2004-500895) is gratefully acknowledged. J.S.A. acknowledges support from MEC, GV and UA (RyC2137/06). A.S.A. acknowledges a Ph.D. fellowship from MEC. References [1] Volatile Organic Compounds, US Environmental Protection Agency, EPA ID: E761346. [2]. [3] Method for eliminating ethanol in exhaust gas, US Patent 6423534. [4] M. Ardjmand, A. Safekordi, S. Farjadfard, Int. J. Environ. Sci. Technol. 2 (2005) 69. [5] P. Christen, F. Domenech, G. Michelena, R. Auria, S. Revah, J. Hazard. Mater. 89 (2002) 253. [6] F. Cioci, R. Lavecchia, M.M. Ferranti, Biotechnol. Tech. 11 (1997) 893. [7] B. Bialobok, J. Trawczynski, W. Mista, M. Zawadzki, Appl. Catal. B: Environ. 72 (2007) 395. [8] H. Teng, C.-T. Hsieh, Ind. Eng. Chem. Res. 37 (1998) 3618. [9] M.A. Lillo-Ródenas, D. Cazorla-Amorós, A. Linares-Solano, Carbon 43 (2005) 1758. [10] M.A. Lillo-Ródenas, A.J. Fletcher, K.M. Thomas, D. Cazorla-Amorós, A. LinaresSolano, Carbon 44 (2006) 1455. [11] C.L. Mangun, M.A. Daley, R.D. Braatz, J. Economy, Carbon 36 (1998) 123. [12] M.-C. Huang, C.-H. Chou, H. Teng, Aiche J. 48 (2002) 1804. [13] Y.-C. Chiang, P.-C. Chiang, C.-P. Huang, Carbon 39 (2001) 523. [14] R.A. Jones, F.H. Tezel, J. Thibault, J.S. Tolan, Int. J. Energy Res. 31 (2007) 1517. [15] H. Marsh, F. Rodriguez-Reinoso, Activated Carbon, Elsevier, London, 2006. [16] F. Rodríguez-Reinoso, J. Garrido, J.M. Martín-Martínez, M. Molina-Sabio, R. Torregrosa, Carbon 27 (1989) 23. [17] J. Silvestre-Albero, C. Gómez de Salazar, A. Sepúlveda-Escribano, F. RodríguezReinoso, Colloids Surf. A: Phys. Eng. Aspects 187–188 (2001) 151. [18] IUPAC Recommendations, Pure Appl. Chem. 57 (1985) 603. [19] F. Rodríguez-Reinoso, M. Molina-Sabio, M.A. Muñecas-Vidal, J. Phys. Chem. 96 (1992) 2707. [20] T.J. Bandosz, J. Jagiello, J.A. Schwarz, A. Krzyzanowski, Langmuir 12 (1996) 6480. [21] R.V.R.A. Rios, J. Silvestre-Albero, A. Sepúlveda-Escribano, F. Rodríguez-Reinoso, Colloids Surf. A: Phys. Eng. Aspects 300 (2007) 180. [22] C. Moreno-Castilla, M.V. López-Ramón, F. Carrasco-Marín, Carbon 38 (2000) 1995. [23] R. Denoyel, J. Fernández-Colinas, Y. Grillet, J. Rouquerol, Langmuir 9 (1993) 515. [24] M.T. González, A. Sepúlveda-Escribano, M. Molina-Sabio, F. Rodríguez-Reinoso, Langmuir 11 (1995) 2151. [25] H.F. Stoeckli, Carbon 28 (1990) 1. [26] F. Rodríguez-Reinoso, M. Molina-Sabio, M.T. González, Langmuir 13 (1997) 2354. [27] M.V. López-Ramón, F. Stoeckli, C. Moreno-Castilla, F. Carrasco-Marín, Carbon 38 (2000) 825. [28] P. Ulbig, T. Friese, S. Schulz, J. Seippel, Thermochim. Acta 310 (1998) 217. [29] R.H. Bradley, I. Sutherland, E. Sheng, J. Chem. Soc. Faraday Trans. 91 (1995) 3201. [30] J. Fernández-Colinas, R. Denoyel, Y. Grillet, J. Vandermeersch, J.L. Reymonet, F. Rouquerol, J. Rouquerol, in: A.B. Mersmann, S.E. Scholl (Eds.), Fundamentals of Adsorption III, New York, 1991, p. 261.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Ethanol removal using activated carbon: Effect of porous structure and surface chemistry A. Silvestre-Albero, J. Silvestre-Albero *, A. Sepúlveda-Escribano, F. Rodríguez-Reinoso Laboratorio de Materiales Avanzados, Departamento de Química Inorgánica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain
a r t i c l e
i n f o
Article history: Received 21 May 2008 Received in revised form 2 September 2008 Accepted 17 October 2008 Available online 26 October 2008 Keywords: Activated carbon Ethanol removal Surface chemistry Porous structure VOCs
a b s t r a c t Activated carbons with increasing porosity have been prepared by chemical activation of olive stones using ZnCl2 followed by physical activation with CO2. The development of porosity and surface area with burn-off favours the adsorption capacity for ethanol. However, the total amount adsorbed (g/100 g AC) achieves a maximum for the sample with 30% burn-off, this amount decreasing thereafter. Apparently, for a low boiling point alcohol such as ethanol there is a critical pore size which allows an optimum packing of the adsorbed ethanol molecules. A further broadening of the porosity becomes detrimental due to the decreased overlapping adsorption potential inside the micropores. Incorporation of surface functionalities (oxygen surface groups) on the activated carbon enhances the adsorption capacity through the development of specific interactions between the ethanol molecule and the oxygen surface groups. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction Environmental regulations are forcing the chemical industry to reduce the emissions of volatile organic compounds (VOCs) both in gas and liquid streams [1]. Among the different VOCs, ethanol is considered one of the most abundant. Ethanol was traditionally originated from bakeries, distilleries and foundry plants. However, the recent incorporation of ethanol in gasoline to replace high octane compounds (aromatics) has increased the emissions of unburned ethanol to the atmosphere. Although ethanol is not considered an extremely harmful pollutant, at high concentrations (above 1000 ppm) ethanol can cause severe health problems such as eye, skin and respiratory tract irritation together with negative environmental effects [2]. Removal of VOCs (e.g. ethanol) can be achieved using different approaches: bio-filtration, adsorption, catalytic combustion, etc. [3–7]. Among the different technologies, bio-filtration is probably one of the most widely applied. Bio-filtration is based on the conversation (biodegradation) of the VOC pollutant into a non-toxic compound using micro-organisms immobilized on the surface of a packing bed. Unfortunately, these bio-systems exhibit important drawbacks such as lack of accurate control of the pH, poor adaptability to changes in gas composition, etc. Adsorption has been proposed as an excellent alternative for the removal of volatile organic compounds avoiding the aforementioned drawbacks [8–13]. At this point, it is noteworthy to * Corresponding author. Tel.: +34 965 90 93 50; fax: +34 965 90 34 54. E-mail address: [email protected] (J. Silvestre-Albero). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.10.012
mention that ethanol adsorption is of paramount importance not only for pollution control but also in the production of bio-ethanol from biomass [14]. In fact, ethanol must be completely recovered from the fermentation broth due to its inhibitory character for certain micro-organism used in the fermentation process. Among the different adsorbents, activated carbons are probably the most interesting candidates, not only because of their low cost, but also because of their tuneable textural and chemical properties [15]. Previous studies concerning the removal of volatile organic compounds using activated carbons have shown that both the porous structure and surface chemistry exhibit an important role defining the adsorption properties. Additionally, these effects have been shown to be highly sensitive to the nature of the adsorptive molecule [8– 13]. Unfortunately, many of these studies do not present the exhaustive textural and chemical characterization required to define general trends or they use a wide variety of samples coming from different origins (different inorganic matter content, etc.) which does not facilitate a direct comparison between them. With this in mind, the goal of this work is to study the adsorption of a common volatile organic compound such as ethanol using a series of activated carbons prepared from the same lignocellulosic precursor (olive stones). The effect of the porous structure on the adsorption properties has been analyzed using a series of carbons with increasing burn-off while the effect of surface chemistry (oxygen surface groups) has been studied using post-synthesis modifications, i.e. an oxidation treatment with HNO3 followed by the selective removal of some oxygen surface groups.
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2. Experimental section A series of activated carbons were prepared by combining chemical and physical activation of olive stones as a precursor. Initially, the lignocellulosic precursor (particle size between 1.7 and 2.0 mm) was impregnated with an aqueous solution of ZnCl2 at 358 K during 7 h under stirring. After the impregnation, the sample temperature was increased up to 373 K in order to evaporate part (70%) of the remaining solution, the final sample being filtered and dried overnight at 353 K. The impregnated sample was submitted to a heat treatment (2 K/min) under a flow of N2 (100 cm3/min) at 773 K for 3 h. After cooling down to room temperature, the carbonized sample was washed with diluted HCl acid (10%) until complete removal of ZnCl2, followed by washing with distilled water until pH 7. Finally, the clean sample was carbonized under a N2 flow (100 cm3/min) at 1123 K for 2 h. The as-synthesized sample was labelled AC0. In a second step, the sample AC0 was submitted to physical activation with CO2 (100 cm3/min) at 1098 K using different periods of time, i.e. 20, 40, 60, 72 and 80 h, which corresponds to a burn-off of 22, 30, 48, 58 and 70%, respectively. The samples were labelled AC followed by the activation time used in each case. In order to study the effect of surface functional groups in the adsorption process, sample AC40 was submitted in a subsequent step to an oxidation treatment with HNO3 (6 M) during 1 h at boiling temperature. The oxidized sample (AC40Ox) was washed with distilled water and subsequently heat treated under He (100 cm3/ min) at 673 K and 973 K in order to selectively remove some of the oxygen surface groups. N2 and CO2 adsorption–desorption isotherms were obtained in a home-made fully automated equipment at 77 K and 273 K, respectively. Before the experiment, the samples were outgassed at 423 K for 4 h under vacuum (10 3 Pa). The ‘‘apparent” surface area was obtained using the BET method. The micropore volume (Vo) was deduced from the N2 adsorption data using the Dubinin–Raduskevitch (DR) equation, while the mesopore volume (Vmeso) was obtained as the difference between the total pore volume (Vt) adsorbed at p/po 0.95 and the micropore volume (Vo). The pore volume corresponding to the narrow microporosity (Vn) was obtained after application of the DR equation to the CO2 adsorption data [16]. Temperature-programmed desorption experiments under helium were carried out to evaluate the amount and nature of the oxygen surface groups present in each sample. Thus, 100 mg of sample were placed in a U-shaped quartz reactor on-line coupled to a quadrupole mass spectrometer (Balzer MSC200). The experiments were performed up to 1273 K under a He flow (50 ml/ min) and a heating rate of 10 K/min. The amount of CO and CO2 evolved were quantified after calibration with calcium oxalate (CaC2O4 H2O). Immersion calorimetry measurements into ethanol (Panreac, ethanol absolute partially denatured QP) and ultrapure water were performed in a Tian-Calvet C80D calorimeter at 303 K. A complete description of the experimental set-up can be found elsewhere [17]. Briefly, the sample was outgassed at 423 K for 4 h in a glass tube connected to a vacuum equipment. After the heat treatment the bulb containing the sample was sealed in vacuum and then it was introduced into the calorimetric cell containing the immersion liquid. Once the thermal equilibrium was reached, the glass bulb tip was broken and the wetting liquid was allowed to contact the sample. The heat evolved as a result of this interaction was recorded as a function of time. The integration of the signal, after appropriate corrections (the breaking of the tip (exothermic) and the heat of evaporation of the immersion liquid to fill the empty volume of the bulb with the vapour at the corresponding vapour
63
pressure (endothermic), both contributions being previously calibrated using empty glass bulbs with different volumes), provides the total enthalpy of immersion ( DHimm). Areal enthalpy of immersion (mJ/m2) was calculated from the enthalpy of immersion (J/g) using the ‘‘apparent” BET surface area obtained from the N2 adsorption isotherms at 77 K. Ethanol breakthrough column experiments were performed in a U-shaped reactor connected on-line to a gas-chromatograph (Agilent GC 6890N). In a common experiment, 500 mg of AC were heat treated in clean air (1000 cm3/g) at 423 K and 523 K, in order to remove water humidity. Then, the sample was cooled down to the adsorption temperature (298 K) and it was contacted with an air flow (1000 cm3/min) containing a concentration of 250 ppmv of ethanol (saturation pressure for ethanol at 265.8 K is 93 Pa). After saturation, the weakly adsorbed ethanol was removed and analyzed by passing an air flow (1000 cm3/min) at 298 K, while the strongly adsorbed ethanol was desorbed and quantified by temperature-programmed desorption analysis (up to 573 K). 3. Results and discussion 3.1. Sample characterization 3.1.1. N2 adsorption–desorption isotherms N2 adsorption–desorption isotherms at 77 K for the different activated carbons are shown in Fig. 1a. Additionally, Table 1 reports the textural characteristics obtained from the N2 and CO2
Fig. 1. N2 adsorption–desorption isotherms at 77 K for (a) activated carbons with increasing burn-off (0, 20, 40, 60, 72 and 80 h) and (b) activated carbon AC40 modified by an oxidation treatment with HNO3 (6 M) followed by a thermal treatment at low (473 K) and high temperature (973 K).
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Table 1 Specific surface area (SBET) and total pore volume (Vt) calculated from the N2 and CO2 adsorption isotherms at 77 K and 273 K, respectively, for the different activated carbons. Sample
N2 adsorption
CO2 adsorption
SBET (m2/g)
Vo (cm3/g)
Vmeso (cm3/g)
Vt (cm3/g)
Vo/Vt
Vn (cm3/g)
AC0 AC20 AC40 AC60 AC72 AC80
866 1253 1448 1983 2097 2127
0.33 0.46 0.54 0.70 0.74 0.76
0.04 0.07 0.09 0.23 0.26 0.26
0.37 0.53 0.63 0.93 1.00 1.02
0.89 0.87 0.86 0.75 0.74 0.74
0.26 0.36 0.40 0.45 0.46 0.47
AC40Ox AC40Ox/HT673 AC40Ox/HT973
1264 1319 1444
0.48 0.49 0.53
0.11 0.11 0.12
0.59 0.60 0.65
0.81 0.82 0.82
0.40 0.38 0.38
adsorption isotherms at 77 K and 273 K, respectively. As it can be observed, all carbons exhibit a Type I isotherm according to the IUPAC classification, characteristic of microporous materials [18]. The as-synthesized chemically activated carbon (AC0) exhibits a narrow knee at low relative pressures, suggesting the presence of slightly narrow micropores. This observation is confirmed by comparing the relatively similar values of micropore volume obtained from the N2 and CO2 adsorption isotherms (see Table 1) [16]. It is noteworthy to mention that application of the DR equation to the N2 adsorption isotherms provides the total volume of micropores (those below 2 nm) while the same equation applied to the CO2 adsorption isotherms provides the volume of narrow micropores (those below 0.6–0.7 nm). Thus, in the absence of kinetic restrictions these two values (Vo, from N2 at 77 K, and Vn, from CO2 at 273 K) must be similar for carbons exhibiting narrow and homogeneous micropores, while they become different (Vo > Vn) with the broadening of the porosity. The subsequent activation of AC0 with CO2 gives rise to an increase in the total amount adsorbed, i.e. an increase in the micropore volume, together with a broadening of the knee in the N2 isotherm. Thus, the activation treatment produces both the opening of new micropores together with the broadening and deepening of the existing ones. The broadening of the microporosity is clearly reflected both comparing the difference between Vo and Vn, and also considering the variation of the relative ratio Vo/Vt (percentage of microporosity) with burn-off. In this sense, the percentage of microporosity slightly decreases with the activation degree down to sample AC40, this value decreasing faster to a relatively common value afterwards (together with a noticeable increase in the mesoporous volume). ‘‘Apparent” BET surface area range from 866 m2/g on the chemically activated carbon (AC0) to 2127 m2/g on the sample with 70% burn-off (AC80). Fig. 1b shows the N2 adsorption–desorption isotherms at 77 K for the activated carbon AC40 before and after the oxidation treatment with HNO3, together with the isotherms after the heat treatment in He at 673 K (AC40Ox/HT673) and 973 K (AC40Ox/HT973). The oxidation treatment with a strong oxidizer such as HNO3 produces some changes in the adsorption properties of sample AC40, although its microporous nature is preserved. The oxidized sample (AC40Ox) exhibits a decrease in the total adsorption capacity, in the surface area and in the total pore volume (see Table 1), with no change in the volume of narrow microporosity. Previous studies on activated carbons have shown that the effect of an oxidation treatment with HNO3 highly depends on the nature of the carbon sample, the concentration of nitric acid and the extent (time) of the oxidation process [19–21]. In this sense, the oxidation treatment can modify only the chemical nature of the carbon surface with mainly no effect in the porosity or it can produce the collapse of small pores, thus modifying the porous structure.
A subsequent heat treatment in helium at low temperature (673 K) has mainly no effect on the adsorption properties while, after a high temperature treatment (973 K), the N2 isotherm resembles that of the original sample. These results can be explained by considering the blocking effect of the oxygen surface groups in the oxidized carbon (AC40Ox) which limit the accessibility of the nitrogen molecule to the narrow micropores at the low adsorption temperature (77 K) [19]. A heat treatment at low temperature (673 K), which only removes the most acidic and less stable oxygen surface groups, produces almost no effect on the adsorption isotherm, confirming that these groups are not responsible for the aforementioned restricted diffusion effect. However, a heat treatment at a higher temperature (973 K) causes the removal of an important part of the oxygen surface groups (the less stable groups, evolved as CO2 in temperature-programmed experiments, together with part of the more stable and less acidic oxygen groups, which evolve as CO), and their corresponding carbon atoms (selective gasification) giving rise to a slight increase in the adsorption capacity (mainly mesoporosity) in respect to the original activated carbon, AC40. In summary, these results show that the oxidation treatment of sample AC40 with a strong oxidizer (HNO3) modifies the surface chemistry with mainly no effect on the porosity. The newly created oxygen surface groups located in the pore mouth of the micropores gives rise to diffusional restrictions for the N2 molecule at 77 K. However, a subsequent heat treatment at high temperature (973 K) with He is able to completely recover the adsorption properties and surface area of the original activated carbon. 3.1.2. Temperature-programmed desorption experiments The amount and nature of the oxygen surface groups on the different activated carbons has been analyzed by temperature-programmed desorption experiments. In this sense, Fig. 2 shows the (a) CO2 and (b) CO desorption profiles for the different activated carbons in the 298–1273 K temperature range. Additionally, Table 2 reports the total amount of oxygen surface groups evolved both as CO2 and CO. Although the TPD profile for CO2 is quite complex in all cases, the contribution of the more acidic groups is quite small as compared with the amount of CO evolved (see Table 2). According to Fig. 2a, an increase in burn-off has mainly no effect in the total amount of oxygen surface groups evolved as CO2 (carboxylic acid, lactone, etc.), i.e. the least stable groups. A similar situation occurs for the oxygen surface groups evolved as CO, i.e. the most stable groups. However, in this case the amount of CO evolved exhibits a slight increase after the activation treatment during 20 h, this amount decreasing afterwards. This effect is clearly reflected in the CO/CO2 ratio, which increases continuously with the degree of activation. However, this value is not meaningful at high burn-off due to the very low amount of CO2. Thus, the activation treatment with CO2 at 1098 K slightly favours the development of oxygen surface groups, mainly those evolved as CO, although the total amount is quite small to expect any effect on the adsorption properties. The oxidation treatment with HNO3 (6 M) produces important changes in the surface chemistry. As it can be observed in Table 2, the oxidized sample (AC40Ox) exhibits an important increase in the total amount of oxygen surface groups evolved both as CO2 and CO, this enhancement being higher for the CO2 groups (lower CO/CO2 ratio), in accordance with previous FTIR results [22]. The subsequent thermal treatment at low temperature (673 K) only produces the selective removal of the more acidic and less stable oxygen groups (mainly carboxylic groups), which is clearly reflected by the increase in the CO/CO2 ratio. A further treatment at 973 K produces the suppression of the less stable groups (evolved as CO2) together with an important decrease in the amount of CO.
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Fig. 2. (a) CO2 and (b) CO temperature-programmed desorption profiles for activated carbons (a) AC0, (b) AC20, (c) AC40, (d) AC60, (e) AC72 and (f) AC80.
Table 2 Amount of CO2 and CO evolved during the TPD experiments on the as-synthesized, oxidized and heat treated activated carbons. Sample
CO2
CO
(mmol/g)
(mmol/g)
CO/CO2
AC0 AC20 AC40 AC60 AC72 AC80
0.06 0.08 0.04 0.03 0.01 0.02
0.16 0.38 0.35 0.33 0.16 0.17
2.7 4.7 8.7 11 16 8.5
AC40Ox AC40Ox/HT673 AC40Ox/HT973
1.35 0.33 0.05
2.45 2.58 0.72
1.8 7.8 14.4
3.1.3. Immersion calorimetry measurements The heat of immersion of a porous solid into a certain liquid can be used to evaluate both the porous structure and the surface chemistry of the material. In the absence of specific interactions at the solid/liquid interface, the enthalpy of immersion can be regarded as an indirect measurement of the surface area available to a certain molecule [23,24]. Thus, the appropriate selection of the immersion liquid (molecular dimensions) can be used to evaluate the surface area available to each molecule, i.e. the pore size distribution [24,25]. However, the heat of immersion for certain molecules can be altered due to the presence of specific interactions between the solid surface and the immersion liquid [26,27]. In this case, the heat of immersion can be used to evaluate the de-
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gree and the nature of the interactions at the solid/liquid interface, information that can be very useful to predict the usefulness of the solid surface into a certain application, e.g. adsorption process. In this sense, Table 3 reports the enthalpy of immersion into ultrapure water and ethanol at 303 K for the different activated carbons as-synthesized, oxidized and heat treated in He at different temperatures (673 K and 973 K). Interestingly, for all samples the enthalpy of immersion (J/g) into ethanol is higher than that obtained using a molecule with a larger dipole moment such as water (dipole moment 1.69 D for ethanol vs. 1.85 D for water), in accordance with previous observations [20,26,28]. The development of porosity, i.e. an increase in the activation degree, gives rise to an increase in the enthalpy of immersion on a unit weight basis for both liquids. Unfortunately, this tendency, which is affected by the increased surface area available to both liquids with the degree of activation, does not permit a direct comparison between the different samples. In this sense, the areal enthalpy of immersion, i.e. the heat of immersion per m2, avoids the aforementioned drawback and allows the comparison of surfaces with different porous structure, i.e. with a different development of porosity. As expected, the areal enthalpy of immersion is quite similar for all activated carbons. Thus, the degree of interaction per m2 for water and ethanol is scarcely affected by the development of the porosity. Only samples above 30% burn-off (AC40) exhibit a slight decrease, this effect being attributed either to the broadening of the porosity (the heat of immersion is enhanced in narrow micropores [29]) or to the lower amount of oxygen surface groups on these samples. The incorporation of oxygen surface groups through an oxidation treatment with HNO3 produces an increase in the enthalpy of immersion for both liquids, this effect being more important for water [19,20,26]. In fact, the ratio DHimm (H2O)/ DHimm (CH3CH2OH) changes from 0.27, in the as-synthesized AC40 sample, to 0.76, in the oxidized carbon (AC40Ox). Similar values have been described in the literature for the interaction of a similar alcohol such as methanol (dipole moment of 1.69 D) and water with the surface of activated carbons and charcoals [26,30]. The large enthalpy of immersion obtained for alcohols (methanol and ethanol) in un-oxidized activated carbons together with the small increase observed after an oxidation treatment clearly suggest that the heat of interaction between both phases, i.e. carbon material/alcohol, must be defined by the carbon surface/ alcohol interactions (dispersion interactions mainly through the hydrocarbon chain), while the oxygen surface groups/alcohol interactions must be less important. The contrary occurs for a polar molecule such as water. In this case, the un-oxidized carbons exhibit a low degree of interaction with water, in accordance with its hydrophobic character. However, the incorporation of surface functionalities (oxygen surface groups) highly enhances the hydrophilic character of the carbon, thus increasing the interaction with water and, consequently, the enthalpy of immersion [19]. Additionally, these primary adsorption sites for water on the carbon surface can act as secondary sites for further adsorption via hydrogen bonding, which in fact will enhance the adsorption enthalpy. This explains the large increase in the DHimm (H2O)/ DHimm (CH3CH2OH) ratio after the oxidation treatment. Calorimetric studies on carbon blacks using a non-polar molecule such as n-heptane and a polar molecule such as water had confirmed the larger degree of interaction with the carbon surface of the hydrocarbon chain in n-heptane compared to water [29]. A subsequent thermal treatment at low (673 K) and high temperature (973 K) gives rise to a progressive decay in the areal enthalpy of immersion, this effect being more evident for water. This behaviour indicates that the degree of interaction between a polar molecule such as water and the carbon surface is highly affected by the total amount of oxygen surface groups, independently of their nature. In fact, a close inspection of the results
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moisture impurities. It is noteworthy to mention that the temperature of this treatment constitutes a critical step defining the adsorption behaviour of activated carbons. In fact, temperatures above 523 K promote the chemisorption of oxygen from the air flow (e.g. 5 wt.% increase at 523 K), thus modifying the surface chemistry and, consequently, the adsorption–desorption properties. To avoid problem with chemisorbed oxygen, a temperature of 423 K was selected for the pre-treatment. Table 4 shows the total amount of ethanol adsorbed together with the amount weakly adsorbed (desorbed at the adsorption temperature) and strongly adsorbed (desorbed at higher temperatures). Interestingly, the amount of ethanol adsorbed (g/100 g AC) increases with the degree of activation up to a maximum on sample AC40, the total amount adsorbed decreasing thereafter. The activation treatment with CO2 gives rise to the development of porosity through the opening of new micropores together with the deepening and enlargement of the existing ones. These effects produce the creation of surface area (see Table 1) which in fact defines the total number of sites available for the adsorption of ethanol. In this sense, Fig. 3 shows the total amount of ethanol adsorbed recalculated per unit surface area available on the activated carbons with different burn-off. The adsorption capacity for ethanol is a decreasing function of the extent of activation, this showing that the microporosity, and more specifically the micropore size and not the total surface area and total pore volume, is the real factor defining the adsorption capacity for ethanol. The decrease in the amount adsorbed per unit area with burn-off should be attributed either to an increase in the diffusion pathway to reach the inner microporosity with the activation degree or, more probably, to the widening of the existing micropores which produce a reduction in the overlapping adsorption potential. Previous studies described in the literature have shown that the effect of the activation degree on the adsorption capacity per unit area of
Table 3 Enthalpy of immersion ( DHimm) into ultrapure water and ethanol for the different activated carbons. Values are referred per unit mass and per unit area, using the ‘‘apparent” surface area (SBET) calculated from the N2 isotherms. Sample
H2O
DHimm (J/g) AC0 AC20 AC40 AC60 AC72 AC80 AC40Ox AC40Ox/HT673 AC40Ox/HT973
CH3CH2OH
DHimm (mJ/m2)
DHimm (J/g)
DHimm (mJ/m2)
18.6 33.1 38.8 43.8 43.8 45.7
22.3 26.5 27.7 22.5 22.0 21.7
89.4 126.5 141.9 171.9 175.5 185.2
107.3 101.2 101.3 88.4 88.2 87.9
127.7 88.9 40.2
97.9 67.3 27.8
168.0 148.4 148.2
132.9 112.5 102.6
reported in Tables 2 and 3 show that the areal enthalpy of immersion into water (DHi) exhibits an approximately linear correlation with the total amount of oxygen surface groups [27,29]. In addition, the intercept of the straight line gives rise to an areal enthalpy of immersion for a ‘‘clean” surface (free of oxygen surface groups) of only DHi = 9 mJ/m2, thus confirming that the specific interactions between the water molecules and the oxygen surface groups are the main component defining the heat of immersion (the interaction with basic groups containing no oxygen, the filling of the micropores and the adsorption on the external surface area being negligible). In the case of ethanol, experimental results denote a better correlation between the areal enthalpy of immersion and the amount of the more acidic and less stable oxygen surface groups (evolved as CO2), although with a larger intercept value (DHi = 103 mJ/m2) [26]. The larger intercept obtained with ethanol highlights the presence of additional highly energetic specific and non-specific interactions with the carbon surface free of oxygen surface groups in the case of alcohols (mainly through the hydrocarbon tail). In this sense, an average immersion enthalpy value of DHi = 107 mJ/m2 was proposed for the dispersion interactions of a hydrocarbon molecule such as n-heptane with the surface of carbon blacks [29]. It is noteworthy to mention that the aforementioned intercept values should be considered as a rough estimation due to the limited number of experimental data available. 3.2. Ethanol adsorption–desorption experiments 3.2.1. Effect of porous structure In a common breakthrough column experiment the activated carbon is contacted with the air flow containing ethanol ([ethanol]0: 250 ppm) until complete saturation. Once the saturation has been achieved, weakly adsorbed ethanol is released by flowing clean air; the remaining more strongly adsorbed ethanol being desorbed by a subsequent temperature-programmed heating. Before the breakthrough column experiment, all samples have been treated in an air flow (1000 cm3/min) in order to remove
Fig. 3. Total amount of ethanol adsorbed per unit surface area (lmol/m2) with burn-off.
Table 4 Amount of ethanol adsorbed (g/100 g AC) at 298 K together with the amount desorbed at 298 K and that desorbed after a heat treatment up to 423 K. Sample
Ethanol adsorbed (g/100 g)
Ethanol desorbed at 298 K (g/100 g)
Ethanol desorbed at 298–423 K
% Weakly adsorbed
% Strongly adsorbed
Ethanol densitya (g/cm3)
AC0 AC20 AC40 AC60 AC72 AC80
5.49 7.13 7.37 6.20 5.57 4.78
5.45 7.12 7.28 6.13 5.54 4.75
0.03 0.00 0.09 0.07 0.04 0.03
99.2 99.9 98.8 98.9 99.3 99.3
0.5 0.1 1.2 1.1 0.7 0.6
0.21 0.20 0.19 0.14 0.12 0.10
a
Amount of ethanol adsorbed per unit volume of narrow micropores (g/cm3).
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carbon materials highly depends on the nature of the adsorptive molecule [8,11,12]. In general, for highly volatile molecules (e.g. acetone, methane, ethane, etc.) the amount adsorbed increases with decreasing pore size while for high boiling point molecules (e.g. pentane, n-hexane, etc.), the amount adsorbed increases with the development of porosity and surface area. Interestingly, the results from Fig. 3 show two distinct behaviours above and below a 30% burn-off. For samples with a low activation degree, the adsorption capacity decreases in a moderate way while for samples above 30% the decrease is slightly more drastic. To further explore the effect of the porous structure in the adsorption of ethanol, Fig. 4 represents the amount of ethanol adsorbed (mmol/g) as a function of (a) the total micropore volume, obtained from the N2 adsorption isotherms (77 K), and (b) the total volume of narrow micropores, obtained from the CO2 adsorption isotherms (273 K). There is in both cases an increase in the amount adsorbed with the pore volume up to an optimum volume around 0.55 cm3/g, for N2, and 0.40 cm3/g, for CO2. This behaviour differs considerable from that observed on larger molecules (benzene and toluene) where a continuous increase in the amount adsorbed with the total micropore volume (Vo) and total volume of narrow micropores (Vn) was observed [9]. Thus, ethanol exhibits an intermediate behaviour between that exhibited by highly volatile molecules (e.g. methane) and that exhibited by larger molecules (e.g.
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benzene). As it can be observed in Fig. 5, for activated carbons with low burn-off the development of porosity is mainly associated with the opening of new micropores together with the deepening of the exiting ones (see also Table 1), thus giving rise to an increased adsorption capacity for ethanol. However, an activation degree above 30% mainly produces the widening of the existing micropores (compare the micropore volume deduced from N2 and CO2 in Table 1 and Fig. 5), together with the development of some mesoporosity, this effect being detrimental for the adsorption of ethanol. Thus, the decreased adsorption capacity above 30% burn-off (sample AC40) must be explained due to the decreased overlapping adsorption potential in larger micropores. This statement is corroborated by comparing the total amount of ethanol adsorbed per unit volume of narrow micropores (g/cm3), which is an indirect measurement of the packing density of the adsorbed phase (see Table 4). These values show that up to 30% burn-off the density of adsorbed ethanol is quite similar and close to the value of 0.20– 0.21 g/cm3, typically present on chemically activated carbons with a narrow micropore size distribution. However, a further development of the porosity, i.e. a widening of the microporosity above a critical pore size, becomes detrimental for the packing density and, consequently, for the adsorption capacity. Interestingly, the absence of a correlation between the amount adsorbed (mmol/g) and the volume of narrow micropores (Vn) (see Fig. 4b) clearly highlights that the critical micropore size for ethanol adsorption must be slightly below the one for CO2. It is noteworthy to mention that on activated carbon materials, mainly all the ethanol adsorbed (>98%) can be easily removed at room temperature (298 K), thus favouring its regeneration. In summary, breakthrough column experiments show that ethanol adsorption at 298 K on activated carbons is highly sensitive both to the volume of narrow micropores (Vn) and to the micropore size distribution. Thus, the development of porosity favours the adsorption capacity for ethanol up to an optimum porous structure which combines a narrow micropore size distribution (high packing density) together with a highly developed narrow microporosity (Vn). A further broadening of the microporosity above the critical micropore size becomes detrimental for the adsorption process due to the decreased packing density of the adsorbed ethanol molecules, i.e. decreased overlapping adsorption potential in larger micropores. 3.2.2. Effect of surface chemistry As expected, the incorporation of oxygen functionalities on the carbon surface after an oxidation treatment highly enhances the adsorption of a polar molecule such as ethanol (see Table 5) [19].
Fig. 4. Total amount of ethanol adsorbed (mmol/g) as a function of (a) total micropore volume (Vo, obtained from the N2 adsorption isotherms at 77 K) and (b) volume of narrow micropores (Vn, obtained from the CO2 adsorption isotherms at 273 K).
Fig. 5. Evolution of the micropore volume (V0), mesopores volume (Vmeso), volume of narrow micropores (Vn) and volume of ethanol adsorbed with burn-off.
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Table 5 Amount of ethanol adsorbed (g/100 g AC) at 298 K together with the amount desorbed at 298 K and that desorbed after a heat treatment up to 423 K, for sample AC40 assynthesized and modified by oxidation and thermal treatments. Sample
Ethanol adsorbed (g/100 g)
Ethanol desorbed at 298 K (g/100 g)
Ethanol desorbed at 298–423 K (g/100 g)
% Weakly adsorbed
% Strongly adsorbed
Ethanol densitya (g/cm3)
AC40 AC40Ox AC40Ox/HT673 AC40Ox/HT973
7.37 9.76 8.15 7.07
7.28 8.96 7.94 6.99
0.09 0.79 0.21 0.08
98.8 91.8 97.4 98.8
1.2 8.1 2.6 1.2
0.19 0.25 0.21 0.19
a
Amount of ethanol adsorbed per unit volume of narrow micropores (g/cm3).
The additional carbon/ethanol interactions through the newly created oxygen surface groups favour the packing of adsorbed ethanol molecules (density of ethanol 0.25 g/cm3) on narrow micropores. However, these specific interactions inhibit the complete desorption (regeneration) of ethanol at room temperature (91% recovery). The selective removal of the less stable and more acidic oxygen surface groups during the thermal treatment in He at 673 K gives rise to an important decrease in the total amount of ethanol adsorbed. Interestingly, the removal of these groups favours the desorption of ethanol at room temperature (>97% recovery). A subsequent treatment at 973 K exhibits a similar effect with a total adsorption capacity very similar to that exhibited by the original sample (AC40). In summary, the presence of oxygen surface groups on activated carbons favours the adsorption of ethanol. However, the presence of specific interactions between the ethanol molecule and the newly created surface functionalities becomes detrimental for the subsequent regeneration of the carbon adsorbent at room temperature. Selective removal of the oxygen surface groups shows that: (i) the adsorption capacity is defined by the total amount and not by the nature of the oxygen surface groups, i.e. the amount of ethanol adsorbed decreases gradually with the total amount of oxygen surface groups, and (ii) the regeneration process is more sensitive to the nature of the oxygen surface groups, in accordance with the calorimetric data (ethanol interacts more strongly with the more acidic and less stable oxygen surface groups). 4. Conclusions Ethanol removal at 298 K has been studied using a series of activated carbons with increasing burn-off. Breakthrough column experiments show that the amount of ethanol adsorbed (g/100 g) increases with the activation degree up to a maximum on sample AC40 (7.4 g/100 g AC), the amount adsorbed decreasing thereafter. Apparently, the adsorption of ethanol on activated carbons exhibits a critical micropore size which favours an optimum packing of adsorbed ethanol molecules, i.e. probably a pore size able to accommodate two adsorbed layers of ethanol. Interestingly, activated carbons can be easily regenerated by passing an air flow at room temperature (more than 98% desorption/recovery). Incorporation of oxygen surface groups on the carbon structure enhances the adsorption capacity for a polar molecule such as ethanol. Selective removal of these surface functionalities highlights that all oxygen surface groups (those evolved as CO2 and CO in TPD experiments) participate in the adsorption process.
Acknowledgments Financial support from MEC (Projects NAN2004-09267-C03-03 and MAT2007-61734) and the Network of Excellence Insidepores (NMP3-CT2004-500895) is gratefully acknowledged. J.S.A. acknowledges support from MEC, GV and UA (RyC2137/06). A.S.A. acknowledges a Ph.D. fellowship from MEC. References [1] Volatile Organic Compounds, US Environmental Protection Agency, EPA ID: E761346. [2]