Environ. Sci. Technol. 2007, 41, 4761-4766
Applications of Pore-Expanded Mesoporous Silica. 7. Adsorption of Volatile Organic Compounds RODRIGO SERNA-GUERRERO AND ABDELHAMID SAYARI* Centre for Catalysis Research and Innovation (CCRI), Department of Chemical Engineering and Department of Chemistry, University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada
Three different varieties of mesoporous silicas were synthesized by varying the postsynthesis treatment of an assynthesized ordered mesoporous material type MCM-41. The resulting materials consisted of a purely siliceous MCM41, a pore-expanded MCM-41 (PE-MCM-41C), and a surfactant-laden pore-expanded MCM-41 (PE-MCM-41E) and were evaluated as adsorbents for two types of volatile organic compounds, i.e., chlorinated and aromatic hydrocarbons. Values of heat of adsorption and Henry’s law constant were determined by pulse chromatography. Additionally, adsorption capacities were calculated with a dynamic method using breakthrough curves for single components in dry and humid environments. The surfactantcontaining material exhibited good compatibility with chlorinated compounds in terms of heat of adsorption and efficiency in gaseous streams containing moisture. Purely siliceous mesoporous materials, i.e., MCM-41 and PEMCM-41C, were more selective toward aromatic hydrocarbons but also gave rise to exceptionally strong adsorption.
Introduction Emissions of volatile organic compounds (VOCs) are one of the most acute environmental problems currently faced by the chemical industry. VOCs are known pollutants that contribute to the production of smog, the greenhouse effect, and ozone layer depletion, and some are considered toxic and hazardous for human health (1, 2). Nonetheless, VOCs are extensively used as solvents, refrigerants, aerosol propellants, and raw materials, resulting in a steadily growing demand and production (3, 4). Due to the general increase in environmental awareness by governments and populations alike, regulations worldwide have grown stricter, compelling industry to substantially decrease its VOC emissions (5, 6). This has resulted in a demand for environmentally friendly technologies to minimize the release of VOCs into the atmosphere. Among the different alternatives available to overcome this problem, separation technologies with no chemical degradation (e.g., adsorption) are attractive, especially in cases where the organic pollutant to be recovered is valuable (7). Ordered mesoporous silicas offer structural and chemical characteristics that make them potentially useful for adsorption separation technologies. Some of their characteristics like high surface area, large pore volume, and good me* Corresponding author phone: (613) 562-5483; e-mail: abdel.
[email protected]. 10.1021/es0627996 CCC: $37.00 Published on Web 05/31/2007
2007 American Chemical Society
chanical stability are greatly appreciated in any adsorbent. Additional features like narrow pore size distribution, highly ordered pore structure, and the possibility to tailor their pore size to target specific applications make them even more attractive. The most dominant mesoporous silica type, namely MCM-41, is characterized by a 2-dimensional hexagonal array of cylindrical pores with a narrow pore size distribution (8, 9). A schematic representation of a popular route for the synthesis of MCM-41 silica is presented in Figure 1 (Material C). According to previous studies, MCM-41 silicas exhibit a hydrophobic nature (10). Hydrophobicity is particularly attractive for the separation of organic compounds in industrial air streams, since water vapor is often present in them and it has been proven that water adsorbs competitively on commercial adsorbents resulting in a diminished capacity for the targeted adsorbates (11). It is also expected that an adsorbent with a hydrophobic nature will exhibit a good compatibility with nonpolar substances (12). To further enhance the hydrophobic character of MCM-41 silica, some authors have proposed the use of mesoporous silica whose surface (13) or framework (14) has been functionalized with organic species, expecting to produce an adsorbent with improved selectivity toward hydrophobic substances. Among the various approaches to produce these organically modified silica mesostructures, the most straightforward method is the use of the as-synthesized MCM-41 material (i.e., before template removal, represented by material “B” in Figure 1). Zhao et al. (15) and Miyake et al. (16), for example, reported that as-synthesized MCM-41 exhibits higher adsorption capacity for chlorinated hydrocarbons and phenol than calcined MCM-41 silica in aqueous environments. However, nitrogen adsorption measurements have shown that the assynthesized MCM-41 is a nonporous material (17, 18), which brings about serious diffusion limitations (19) and results in comparatively lower capacities during gas-phase separations, (20, 21). Ideally, to achieve high capacity and adsorption rate, the functionalized material should exhibit a surface layer of alkyl chains combined with high porosity. This was achieved in earlier works by our group (18, 22) using a postsynthesis pore expansion technique. In this approach, as-synthesized MCM-41 undergoes a hydrothermal treatment in the presence of dimethyldecylamine, generating a material with larger pores containing the surfactant template and the swelling agent, as represented by “E” in Figure 1. The swelling agent is then selectively extracted with ethanol, producing a mesoporous material with a surfactant layer on its surface and large void channels for the adsorbate molecules to diffuse freely (material “F” in Figure 1), making it extremely promising as an adsorbent of nonpolar substances. Calcination of E or F affords material G, which, depending on the pore expansion condition, may exhibit pore sizes up to 20 nm and 3.5 cm3 g-1 in pore volume without any significant loss in surface area. However, postsynthesis pore expansion is accompanied by a gradual decrease in the ordering of the pore system, particularly beyond 6 nm in pore diameter. This study aims at exploring the possibility to produce mesoporous materials with exploitable characteristics as adsorbents of VOCs in air streams. For that reason, Materials C, F, and G (Figure 1) were evaluated through measurements of heat of adsorption (-∆Ha) and adsorption capacity (q). The heat of adsorption represents the strength of interactions between the adsorbent and the adsorbed species and can be used to gauge the compatibility between them. The value of q provides a measurement of the total amount of adsorbate that can be adsorbed by a particular material under specific conditions of temperature and concentration. Finally, the VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic representation of the synthesis routes used for the mesoporous materials hereby studied. adsorption capacity (qh) in a stream of VOC and nitrogen with a relative humidity of 7% was estimated to evaluate the efficiency of the adsorbents under a more realistic environment. Molecules corresponding to two of the VOC groups that generate profound environmental concern were chosen as adsorbates: dichloromethane to represent chlorinated hydrocarbons and benzene to represent aromatic compounds.
Materials and Methods Materials. The ordered mesoporous materials were prepared using Cab-O-Sil fumed silica powder from Cabot as the silica source, cetyltrimethylammonium bromide (CTAB, Aldrich) as template, tetramethyl ammonium hydroxide (TMAOH 25% balance water, Aldrich) for pH adjustment, and dimethyldecylamine (DMDA 97% purity, Aldrich) as postsynthesis pore expander agent. All volatile organic compounds used were of high-purity liquid chromatographic grade: dichloromethane (99.96%, EM-Merck), chloroform (99.8%, BDH), benzene (99.9%, Sigma-Aldrich), and toluene (99.8%, Alrich). Synthesis of Mesoporous Materials. The mesoporous materials were prepared based on the procedure described elsewhere (18, 23). An amount of 578.6 g of TMAOH 25% was diluted in 5500 g of water under vigorous stirring in an 8 L stainless steel vessel. Cetyl-trimethylammonium bromide (820 g) was subsequently added and stirred for 15 min, after which 328.4 g of Cab-O-Sil fumed silica was added. The mixture obtained had the following composition: 1.0 SiO2: 0.343 TMAOH:0.41 CTAB:57 H2O. After mixing for 30 more minutes at ambient temperature, the resulting gel was placed in an oven and stirred at 100 °C under autogenous pressure for 40 h. The obtained material was thoroughly washed with water, filtered, and dried at ambient conditions. A sample of this product was calcined for 5 h at 550 °C under nitrogen 4762
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flow during the temperature ramp then under flowing air when the temperature reached 550 °C. This template-free material was labeled MCM-41. Another sample of as-synthesized MCM-41 (i.e., before calcination) underwent a pore expansion procedure consisting of a hydrothermal treatment using dimethyldecylamine (DMDA) as expander agent. An emulsion was prepared by adding 437.5 g of DMDA in 5250 g of water under vigorous stirring at ambient temperature. Subsequently, 350 g of assynthesized MCM-41 was added and the mixture was kept under stirring for 15 more min. The resulting suspension was heated in a stainless steel closed vessel at 120 °C under continuous stirring for 72 h. After this time, it was washed with water, filtered, and dried in ambient air. Following the procedure described by Sayari et al. (18), DMDA was selectively extracted by washing the material with ethanol twice using a ratio of 18.5 mL and 9.7 mL per g of as synthesized MCM-41 in the first and second extractions, respectively. The material was subsequently filtered and dried in air. The resulting product will be referred to as PE-MCM41E where “PE” and “E” indicate, respectively, that the material was pore-expanded and selectively solvent-extracted. A sample of the material obtained after pore expansion was calcined at 550 °C for 5 h to remove both expander and template surfactants and was labeled PE-MCM41C, with “C” indicating that the pore-expanded material was calcined. To avoid excessive pressure drop in the adsorption column, pellets were produced by pressing the materials using a hydraulic press at a maximum pressure of 450 kgf cm-2. The solids obtained were subsequently crushed and sieved between nets with pore openings of 40 and 60 mesh. Before each adsorption experiment, the adsorbent was regenerated overnight in flowing nitrogen at 50 °C for PEMCM-41E and 200 °C for MCM-41 and PE-MCM-41C. All measurements performed in the present work used adsorbents in pelletized form. Characterization. The materials produced were characterized by nitrogen adsorption at 77 K using a Micromeritics ASAP 2020 automated volumetric instrument. Prior to each analysis, the purely siliceous materials (i.e., MCM-41 and PE-MCM-41C) were treated at 100 °C under high vacuum (1 × 10-5 Torr), while PE-MCM-41E was evacuated at 1 × 10-5 Torr at ambient temperature to avoid thermal degradation of the organic layer on its surface. The specific surface area (SA) was determined using the Brunauer-Emmett-Teller (BET) method and the pore size distribution was calculated using the KJS approach proposed by Kruk, Jaroniec, and Sayari (24). The pellet density (Fp) was measured with a Micromeritics DynaPyc 330 pycnometer using ultrahigh purity (UHP) helium as displacement gas. The bed apparent density (Fb) was determined experimentally by weighting the amount of adsorbent pellets contained in a vial with a volume of 2 mL with the aid of an analytical balance. The adsorption isotherms for dichloromethane and benzene were generated using a TA Q-500 thermogravimetric analyzer (TGA). For each point in the isotherm a stream with known concentration of VOC in nitrogen was fed into the TGA sample chamber. The amount adsorbed was estimated at the time when the weight of the sample was constant and hence equilibrium could be assumed. Heat of Adsorption and Henry’s Law Constant. The heat of adsorption (-∆Ha) and the Henry’s Law constant (Kp) were determined using pulse chromatography, with the experimental setup represented schematically in Figure S1 in the Supporting Information section. Nitrogen UHP grade was fed into the system through two MKS type M100-B mass flow controllers. Nitrogen flowing at 50 mL min-1 was used as carrier gas. A second line of nitrogen was directed into a glass saturator containing 50 mL of the VOC to be analyzed
TABLE 1. Structural Parameters of Mesoporous Silicas Determined by Nitrogen Adsorption Measurements at 77 K SA (m2 g-1)
Vp (cm3 g-1)
dP Gp Gb (nm) (g cm-3) (g cm-3)
MCM-41 1086 0.86 3.5 PE-MCM-41E 379 (689)a 0.83 (1.51)b 7.7 PE-MCM-41C 1119 2.35 9.2 -1 a Units are m2 per g of silica (m2 g silica ). silica (cm3 gsilica-1).
b
2.29 1.44 2.54
0.55 0.80 0.37
Units are cm3 per g of
to produce a mixture of organic vapor and nitrogen. The saturator was placed inside a bath of water and ethylene glycol with controlled temperature to produce a stream with known concentration of VOC (C0). Both lines were connected to a six-port two-position valve with an actuator controlled electronically to switch between “load” and “inject” positions. When the valve is in “load” position, the gaseous mixture flows through a 0.025 mL loop, while the pure nitrogen gas flows directly into a fixed bed column. After the valve is switched to the “inject” position, the carrier gas flows through the fixed volume loop, carrying with it a pulse of the VOC into the packed column. The column used was made of stainless steel and had an inner diameter of 0.36 cm and a packed length of 12 cm. To control the adsorption temperature, the column was placed inside an electric oven with controllable temperature. The column effluent was continuously monitored with a flame ionization detector (FID). All data produced by the FID were recorded using a National Instruments SBC-68 DAQ electronic interface and processed with a program developed in Labview 7.1. Henry’s law constants (Kp) can be estimated from the resulting profiles of concentration at the column outlet (CA) versus time (t) as detailed in the Supporting Information. By estimating the values of Kp at different adsorption temperatures, an Arrhenius type equation can be generated where the slope of the function is proportional to the heat of adsorption as expressed by eq 1 (25-28):
lnKp ) lnA -
∆Ha (T)-1 R
(1)
Adsorption Capacity. The adsorption capacity (q) was determined from breakthrough curves generated dynamically with the experimental setup shown schematically in Figure S1. A line of UHP nitrogen was used for preparation of the adsorbent flowing at 50 mL min-1 for at least 10 h. A second line of nitrogen was used as carrier gas flowing at 30 mL min-1 into a glass saturator containing the VOC to be analyzed. The saturator was located inside a water-ethylene glycol bath with temperature control. The temperature of the saturator was adjusted at -4 °C for dichloromethane and 6 °C for benzene to generate a VOC partial pressure of P ) 0.10 × P0, where P0 is the vapor pressure of saturation at the temperature of the fixed bed column, i.e., 50 °C. Both lines were directed to a four-way two-position valve to feed the fixed bed column with pure nitrogen or a VOC/N2 mixture. The column outlet was monitored with an FID and the produced data were managed using a National Instruments data acquisition card and Labview 7.1 software. To evaluate the selectivity of the adsorbents toward VOCs in the presence of moisture, the adsorption capacities (qh) for each VOC in streams containing water vapor were determined. For such analyses, a third line was used to generate a humid nitrogen stream by flowing 15 mL min-1 nitrogen into a glass saturator containing water. The temperature of the saturator was maintained at 5 °C using a water-ethylene glycol bath producing a relative humidity of 7% at the adsorption temperature, corresponding to a typical humidity in industrial
gas streams after moisture removal by other inexpensive methods (e.g., cooling towers). The stream of humid nitrogen was mixed with the VOC stream before contacting the adsorbent, resulting in a stream containing N2, VOC, and water with a total flow rate of 30 mL min-1 in order to feed both adsorbates simultaneously into the fixed bed column. Temperatures in the saturator were modified accordingly to maintain a VOC partial pressure of 0.10 P0 at the column inlet even after mixing with the humid nitrogen stream. The value of q was calculated as a function of the molar flow of VOC (FA), the mass of adsorbent packed in the column (W), and the stoichiometric time (tq) determined from the experimental breakthrough curves:
q)
FAtq W
(2)
The time tq is estimated according to eq 3 (29):
tq )
∫ t
(
1-
)
CA dt - tD C0
(3)
where CA and C0 are the concentrations of the adsorbent at the column downstream and upstream, respectively, and tD is the dead time of the system estimated experimentally to be 30 s.
Results and Discussion Characterization. The structural properties determined experimentally are compiled in Table 1. The resulting adsorption isotherms can be consulted in the Supporting Information section as Figure S2. In all cases, isotherms correspond to Type IV according to the IUPAC classification, with the characteristic N2 condensation and evaporation steps of mesoporous materials with narrow pore size distributions (9, 10). It is interesting to notice that while PE-MCM-41C showed a much larger pore volume (Vp) than MCM-41, the hybrid silica PE-MCM-41E exhibited a Vp similar to that of MCM-41, despite of its pore enlargement. These results suggest there is space occupied inside the pores of PE-MCM41E that can be attributed to the surfactant molecules retained, but also that a significant porosity was maintained. All three materials had relatively narrow pore size distributions with mean pore diameters (dP) of 3.5, 7.7, and 9.2 nm for MCM-41, PE-MCM-41E, and PE-MCM-41C, respectively. With respect to SA, very high values were obtained for both MCM-41 (1086 m2 g-1) and PE-MCM-41C (1119 m2 g-1). In comparison, the SA value of 379 m2 g-1 presented by PEMCM-41E was smaller and may appear disadvantageous to its adsorptive properties, since higher SA is generally related to better performance. However, it must be born in mind that the aforementioned values of surface area and pore volume are based on the total mass of material which, in the case of PE-MCM-41E, includes both the silica and the occluded cetyltrimethyl ammonium (CTA) cations which, according to thermogravimetric data (Figure S3 in Supporting Information), represent approximately 45% of the total weight of the material. For a more appropriate comparison between the structural properties of the different materials, analysis should be performed using a per-gram-of-silica basis. Accordingly, values of SA and Vp of PE-MCM-41E based on the silica content are shown in Table 1. As can be seen, while the SA of PE-MCM-41E on silica content base is still the lowest, its value is in a range closer to those for MCM-41 and PEMCM-41C. The adsorption isotherms for benzene and dichloromethane are presented in Figure 2. For comparison, the adsorption isotherms of different types of activated carbon are also shown (30). In agreement with literature data, activated carbon shows higher capacity than mesoporous VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Adsorption isotherms of (a) dichloromethane and (b) benzene on mesoporous adsorbents. Open symbols represent values obtained by static measurements (TGA) and closed symbols represent values obtained by dynamic measurements (breakthrough curves). Isotherms for activated carbon A10 (- ‚ -), CQS (- ‚‚ -), and BPL (- - - -) are included. silicas at low concentrations as a result of its microporous structure. However, at high concentrations, mesoporous adsorbents outperform activated carbon (31). It is interesting to notice that, unlike other studies using non-expanded assynthesized MCM-41, PE-MCM-41E presents a higher capacity at equilibrium than MCM-41 at high adsorbate concentrations. This unique behavior can be attributed to the existence of a porous structure in PE-MCM-41E and represents a clear advantage with respect to the assynthesized adsorbents explored by other authors. Heat of Adsorption. The results obtained for halogenated hydrocarbons are presented in Figure 3a as plots of Kp vs T-1. As shown, PE-MCM-41E was found to be the adsorbent with the highest value of -∆Ha. The stronger interactions between the hybrid silica and the organic adsorbates can be attributed to the enhanced hydrophobic character of the adsorbent, resulting in a better compatibility with nonpolar substances. To corroborate the results with dichloromethane, additional experiments were performed using chloroform as adsorbate. As shown in Figure 3a, the tendencies of -∆Ha and Kp observed with dichloromethane were reproduced with chloroform, evidencing the compatibility between PE-MCM-41E and chlorinated hydrocarbons. It is known that chloroform is less polar than dichloromethane (31), so it is reasonable to suggest that the comparatively higher values of -∆Ha estimated for chloroform are a result of a preferential adsorption for nonpolar substances exhibited by all three adsorbents. The particular adsorptive characteristics of each mesoporous material hereby studied were further evidenced with the experiments performed with aromatic hydrocarbons. As seen in the plots of Kp vs T-1 presented in Figure 3b, contrary 4764
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to the results with halogenated compounds, PE-MCM-41E presented the lowest values of -∆Ha and Kp for the adsorption of benzene. A reasonable explanation can be based on the work of Inel et al. (32) where it was suggested that the hydroxyl groups present in the silica surface behave as weak acid sites interacting strongly with the π-electrons of unsaturated organic molecules. Such interactions would be inhibited on PE-MCM-41E due to the surfactant layer on its surface. The presence of this chemical interaction is also consistent with the values of -∆Ha, which are in a much higher range for the adsorption of benzene over MCM-41 and PE-MCM-41C than the adsorption of dichloromethane or chloroform. Cyclohexane, which is a six-carbon ring like benzene but with no π-electrons, produced values of -∆Ha comparable to those for dichloromethane (as detailed in Supporting Information, Figure S4). To corroborate these results, experiments were performed using toluene as a second representative of aromatic hydrocarbon. The results in Figure 3b show how the tendency of values of ∆Ha and kp are consistent in the cases of toluene and benzene. Analogous to the results obtained with chlorinated compounds, the less polar aromatic molecule (i.e., toluene) produced stronger bonds with the adsorbents as evidenced by higher values of -∆Ha than benzene. The results hereby obtained are indication of an effective change in the adsorbate-adsorbent interaction mechanism between PE-MCM-41E and its purely siliceous counterparts. It should be noted that the high values of -∆Ha exhibited by MCM-41 and PE-MCM-41C with aromatic compounds might not be suitable for an application involving regeneration of the adsorbent. Indeed, because of the strong adsorbentadsorbate interactions, excessive energy would be required to regenerate the adsorbent, making their use economically unattractive. Adsorption Capacity. Table 2 presents values of adsorption capacity obtained under dry (q) and humid environments (qh), as well as the adsorption efficiency in the presence of water (qh/q). Typical breakthrough curves obtained experimentally are presented in Figure S5 in the Supporting Information. As mentioned before, to provide a fair comparison between the purely siliceous adsorbents and PEMCM-41E values of capacity on a per-gram-of-silica basis were included in Table 2 and are used in the following analysis. The high compatibility between PE-MCM-41E and chlorinated hydrocarbons previously observed resulted in an adsorption capacity of 1.76 mmol/gsilica for dichloromethane, a value close to that of MCM-41 despite its comparatively lower SA. MCM-41 offered the highest q for dichloromethane followed by PE-MCM-41C, which reflects the adsorption capacity of each material at equilibrium. As seen in the adsorption isotherms presented in Figure 2, at the same partial pressure of VOC used for the breakthrough curves experiments (i.e., P ) 0.10 × P0), q at equilibrium is also in the order MCM-41 > PE-MCM-41C > PE-MCM-41E. The experiments performed under humid conditions lend further support to the good compatibility between the hybrid silica and the chlorinated adsorbates. As seen from the results presented in Table 2, PE-MCM-41E offers the highest qh for dichloromethane in the presence of water vapor with 1.56 mmol/gsilica. The latter as a result of an adsorption efficiency of 89% with respect to q in dry stream presented by PEMCM-41E was, as opposed to 84% for PE-MCM-41C and only 80% for MCM-41. The higher efficiency for dichloromethane in a humid environment offered by the hybrid silica and the aforementioned compatibility according to -∆Ha makes it an attractive adsorbent for potential applications involving separation of chlorinated hydrocarbons. On the other hand, the extremely strong compatibility between aromatic hydrocarbons and silica surfaces is
FIGURE 3. Arrhenius plots for the adsorption of (a) dichloromethane (closed symbols) and chloroform (open symbols) and (b) benzene (closed symbols) and toluene (open symbols). The values presented in the plot correspond to the heat of adsorption estimated from the closest set of data.
TABLE 2. Adsorption Capacity of Mesoporous Silicas for Selected VOCs at a Relative Pressure of P/P0 ) 0.10 in mmol per gram of Silica (Values in Parentheses are Expressed in mmol per gram of Adsorbent) and Stoichiometric Breakthrough Time in Seconds dichloromethane adsorbent
tq
MCM-41 289 PE-MCM-41C 160 PE-MCM-41E 212
q
qh
benzene
qh/q
tq
q
qh
qh/q
1.88 1.48 0.80 1331 2.43 2.10 0.87 1.55 1.30 0.84 767 2.07 1.73 0.84 1.76 1.56 0.89 568 1.40 1.13 0.81 (0.97) (0.86) (0.77) (0.62)
evidenced again in the results of adsorption capacity for benzene, with values of q for PE-MCM-41C (2.07 mmol g-1) and MCM-41 (2.43 mmol g-1) much higher than the capacity of PE-MCM-41E (1.54 mmol/gsilica). It is interesting to observe that the values of q for benzene are remarkably similar to the capacities estimated at equilibrium. If, as previously suggested, there are strong interactions between the silica surface and the π-electrons of aromatic hydrocarbons, it is possible that the adsorption capacity for aromatic compounds is strongly influenced by the surface chemistry in purely siliceous adsorbents minimizing the effect of resistances present in dynamic processes. In terms of adsorption
capacity, MCM-41 appears to be the offer the best performance of the adsorbent studied. However, it must be kept in mind that although the capacity of PE-MCM-41E appears unfavorable, as previously suggested its lower demand of energy for regeneration could be favorably exploited in cyclic processes. With respect to the experiments performed in humid environments, the best efficiency toward benzene in the presence of water was exhibited by MCM-41 with 87% of its original capacity in dry streams. It is interesting to observe that whether for benzene or dichloromethane, PE-MCM41C was the second most efficient adsorbent with a qh to q ratio of 84% in both cases. This suggest that, while MCM-41 and PE-MCM-41E may be suitable to target specific types of organic molecules in humid environments, like chlorinated and aromatic hydrocarbons, respectively, PE-MCM-41C can be a good alternative for more general purposes or when the targeted adsorbate is a mixture of different types of molecules.
Acknowledgments We thank the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Foundation for Innovation (CFI), and the Ontario Research & Development Challenge Fund (ORDCF) for financial support. We thank P. J. E. Harlick for valuable discussions and his help with the VOL. 41, NO. 13, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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experimental setup. A.S. is the Government of Canada Research Chair in Catalysis by Nanostructured Materials.
Supporting Information Available Details on the experimental setup including a schematic diagram, the mathematical background used to determine -∆Ha, nitrogen adsorption isotherms of the adsorbents, thermogravimetric analysis profile for PE-MCM-41E, Arrhenius plots for the adsorption of cyclohexane on mesoporous adsorbents, and typical experimental breakthrough curves. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review November 25, 2006. Revised manuscript received April 2, 2007. Accepted April 15, 2007. ES0627996
