Environ. Sci. Technol. 2003, 37, 4007-4010
MTBE Adsorption on All-Silica β Zeolite SHIGUANG LI, VU A. TUAN, RICHARD D. NOBLE, AND JOHN L. FALCONER* Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424
All-silica β zeolite is shown to be effective for MTBE removal from water. The silica β was prepared directly from gel, and it was more effective than dealuminated β for MTBE adsorption. Water and 2-propanol adsorption isotherms showed that the all-silica β is more hydrophobic than dealuminated β. The amount of MTBE adsorbed on allsilica β increased linearly with MTBE concentration from 9.4 to 590 µg/L.
Introduction Methyl tert-butyl ether (MTBE, C5H12O) is used throughout the United States as a gasoline additive to reduce the levels of pollutants caused by automobile combustion emissions. Both groundwater and surface water have been contaminated with MTBE, primarily by leaks or spills from underground gasoline storage tanks (1). Because MTBE is a hazardous chemical and is generally unpleasant in taste and odor, the U.S. Environmental Protection Agency (EPA) advises that MTBE levels not exceed 20-40 µg/L (2), and the California Department of Health Services issued a maximum contaminant level of 5 µg/L for MTBE (3). Therefore, efficient methods are needed to remove MTBE from contaminated water. Traditional treatment methods for pollutant remediation such as air stripping, carbon adsorption, and aerobic biodegradation do not remove MTBE from water as effectively as they remove other volatile organic compounds (VOCs) (4). A recent study by Anderson (5) showed that hydrophobic, dealuminated mordenite, a 12-membered ring zeolite with 0.65 × 0.70 nm pores, adsorbed MTBE from water better than activated carbon. Anderson equilibrated 5 mg of zeolite powders with 25 mL of aqueous solutions containing 100 µg/L MTBE for 15 min, and dealuminated mordenite removed 96% of the MTBE. The hydrophobicity of zeolites is controlled by the Si/Al ratio in the framework, structural defects, cations present in the structure, synthesis conditions, and post-synthesis treatments (6). The most studied hydrophobic zeolite is silicalite1, which is a 10-membered ring, MFI structure composed of pure silica. The MFI pores are elliptical with diameters of 0.52-0.57 nm, based on X-ray diffraction (XRD) measurements. Thus, MTBE (0.62 nm kinetic diameter (7)) would have difficulty adsorbing into the MFI pore. In contrast, β zeolite possesses a three-dimensional, 12-membered ring, interconnected channel system with pore diameters of 0.71 × 0.73 nm (8). These diameters are similar to those of mordenite. Moreover, Camblor et al. (9) reported that β zeolite could be synthesized in an all-silica form without framework defects. They prepared all-silica β with enhanced crystallinity * Corresponding author telephone: (303)492-8005; fax: (303)4924341; e-mail:
[email protected]. 10.1021/es0264044 CCC: $25.00 Published on Web 08/05/2003
2003 American Chemical Society
and high thermal stability by hydrothermal crystallization of a gel containing tetraethylammonium hydroxide (TEAOH), tetraethyl orthosilicate (TEOS), and fluoride ions at nearneutral pH. Takewaki et al. (10) synthesized a highly crystalline all-silica β by heating TEAOH-impregnated SiMCM-41. 29Si magic angle spinning (MAS) NMR measurements showed that their all-silica β had a small amount of Q3 sites (Si bonded to 3 Si and 1 OH), most of which were siloxy groups that balanced the charge of the tetraethylammonium (TEA) cations. The TEA cations in this all-silica β could be extracted by acetic acid to form a highly hydrophobic all-silica β. Stelzer et al. (11) prepared all-silica β zeolite using the procedure of Camblor et al. (9). Their adsorptions of single components and water/hydrocarbon vapor mixtures indicated that all-silica β is much more hydrophobic than silicalite-1, other comparable 12-membered ring zeolites such as dealuminated Y and EMT zeolites, all-silica mesoporous MCM-41, and activated carbon F300. The all-silica β, therefore, is expected to be more hydrophobic than dealuminated mordenite, and it can be prepared directly (without dealumination). The current study measured the adsorptive properties of β zeolites for MTBE removal from aqueous solutions. Three types of zeolite (H-exchanged Al-containing β (designated as H-β), dealuminated β, and all-silica β) were synthesized and characterized by XRD; inductively coupled plasma (ICP); scanning electron microscope (SEM); and N2, water vapor, and 2-propanol vapor adsorptions.
Materials and Methods Synthesis of β Zeolites. The H-β zeolite powder was prepared by ion-exchanging Na-β powder. To prepare Na-β, a synthesis gel mixture was made by stirring 1.3 g of NaAlO2 (53 wt % Al2O3, 43 wt % Na2O) and 8 g of H2O at room temperature until a clear solution was obtained. Then 42.6 g of TEAOH (40 wt % aqueous solution, Aldrich) in water was added. Next, 38.5 g of TEOS (99.9%, Alfa Aesar) was added to the solution while it was stirred. The resulting mixture was heated at 323 K for 1 h while being stirred to remove ethanol that formed by TEOS hydrolysis. Note that ethanol must be completely removed to avoid formation of amorphous material. The gel was placed in a stainless steel tube (Mott Corporation, 0.5-µm pores), which was placed vertically in a Teflon-lined autoclave. Hydrothermal synthesis was carried out at 393 K for 6 d. The Na-β powder was calcined at 773 K for 8 h and then exchanged three times with NH4NO3 solution (1.5 M) at 353 K for 3 h. The NH4-exchanged β powder was calcined at 773 K for 4 h to obtain the H-form. To prepare dealuminated β zeolite, about 0.5 g of H-β powder was added to a Teflon-lined autoclave containing 20 mL of 60 wt % HNO3 aqueous solution. The mixture was stirred at 353 K for 24 h. The β powder was collected by centrifuge, then washed, dried, and calcined in air at 773 K for 4 h. The dealumination procedure was performed twice. We used a method similar to that reported by Camblor et al. (9) to prepare the all-silica β. A mixture containing 8.8 g of 35% TEAOH, 1.0 g of H2O, and 8.0 g of TEOS was stirred at room temperature in a capped polypropylene bottle. Then 0.86 g of HF was added to the solution while being stirred, and the mixture turned into a white solid gel. Porous stainless steel tubes (Pall Corporation, 4-µm pores) were filled with the gel and then wrapped with Teflon tape. The tubes were capped and then placed vertically in a Teflon-lined autoclave. Hydrothermal synthesis was carried out at 408 K for 4 d while rotating the autoclave at 60 rpm. After synthesis, the powder VOL. 37, NO. 17, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. XRD patterns of β zeolites: (a) uncalcined Na-β, (b) calcined dealuminated β, (c) uncalcined all-silica β, and (d) calcined all-silica β. was washed with distilled water, dried at 373 K, and calcined at 773 K for 8 h to remove the template. Characterization. The crystallinity and phase purity of the β zeolites were determined by powder XRD (Scintag PAD-V diffractometer) with Cu KR radiation. The Si/Al ratio in the zeolite was determined by ICP (ARL, model 3410+). All-silica β powder was also analyzed by SEM (ISI SX-30 instrument) operated at 30 keV. Gas and vapor adsorptions were performed on an Autosorb-1 (Quantachrome Corp. model AS1-C-VP-RGA) system. Prior to the adsorption experiments, the samples were outgassed at 493 K for 5-10 h in a vacuum. Nitrogen adsorption was performed at 77 K to determine the BET surface area and micropore volume. The adsorption isotherms for vapor-phase water and 2-propanol were measured at 293 K. The saturation pressures (po) for water and 2-propanol at 293 K are 2.34 and 4.40 kPa, respectively. MTBE Adsorption. Five milligrams of each β zeolite was added to 25-mL solutions containing 11 000 µg/L of MTBE. In addition, known amounts of the all-silica β powder were placed in solutions containing 830 µg/L of MTBE. One sample was prepared for each condition. All solutions were equilibrated for 30 min while being stirred and then centrifuged. The resulting solutions were poured into 25-mL polypropylene scintillation vials, which were completely filled so the headspace was less than 0.2 mL. The lids were sealed with tape, and the vials were shipped (1 d) for analysis. The samples were kept at 271-277 K and analyzed within 5 d. The MTBE concentrations were measured by a TekmarDohrmann LSC2000/ALS2032 purge-and-trap coupled to a Hewlett-Packard series 5890 gas chromatograph equipped with a photoionization detector. All samples were analyzed once except as indicated in Table 2. Most samples were diluted by 10 times, but some were diluted by 50 or 1000 times, and some were not diluted. Laboratory control samples (LCS), reagent water spiked with MTBE at a known concentration, were analyzed in the same manner as the samples. The percent recovery of the LCS gave an indication of the accuracy. The MTBE detection limit was 2.5 µg/L. MTBE adsorption was also carried out in a flow system; 50 mg of all-silica β powder was placed in a 0.23 cm i.d. Pyrex tube with porous glass frit. An aqueous solution containing 47 µg of MTBE/L was fed to the top of the column, and samples were collected periodically from the bottom of the column. The flow rate was 3.0 mL/h.
Results and Discussion Characterization of β Zeolites. The XRD patterns of zeolite β powders are shown in Figure 1. All peaks for uncalcined 4008
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FIGURE 2. SEM image of all-silica β zeolite powder.
TABLE 1. Properties of β Zeolites β zeolite
Si/Al ratio
H7.1 dealuminated 280 b all-silica
BET surface micropore hydrophobicity area (m2/g) vol (cm3/g) indexa 463 416 434
0.24 0.21 0.23
1.0 1.5 7.0
a The ratio of 2-propanol to water adsorbed at p/p ≈ 0.9. b Analyzed o twice, the relative deviations for BET surface area and micropore volume were (2.3%.
Na-β (Figure 1a) and calcined dealuminated β (Figure 1b) match those reported by Robson (12) with respect to the positions; the peak at 2θ ) 22.4° is typical of the BEA topology. The pattern of uncalcined all-silica β (Figure 1c) is in good agreement with the XRD spectra reported in the literature (9, 11). After the powder was calcined at 773 K for 4 h, the intensity of the peak at 2θ ) 22.4° decreased, whereas the intensity of the peak at 2θ ) 7.5° increased (Figure 1d). The SEM image of the all-silica β zeolite powder in Figure 2 shows intergrown cubic and spherical crystals with particle sizes from 1 to 10 µm. The Si/Al ratios measured by ICP and BET surface areas and micropore volumes measured by N2 adsorption are listed in Table 1. The ICP measurements show that most of the Al was removed by the dealumination procedure. The H-β zeolite had the highest BET surface area and micropore volume. Similar behavior was reported by Stelzer et al. (11). They indicated that the removal of Al atoms in the lattice leads to a contraction of the unit cell. Dealumination has been reported to cause structure degradation and formation of amorphous material for β (13), Y (14), and mordenite zeolites (15); thus, the lower surface area and micropore volume for the dealuminated β is not surprising. The pore volume of our all-silica β (0.23 cm3/g) is comparable to that of all-silica β prepared by Stelzer et al. (0.24 cm3/g) (11). Water vapor adsorption isotherms at 293 K are shown in Figure 3. Dealumination increased the hydrophobicity; the amount of water adsorbed decreased by 30-85% upon dealuminated, even though the surface area only decreased by 10%. The all-silica β was even more hydrophobic than the dealuminated β. At high pressures (p/po ≈ 0.9), the amount of water adsorbed (0.023 g/g) on all-silica β was only 15% of that on dealuminated β (0.15 g/g) and 10% of that on H-β (0.22 g/g). The most hydrophobic all-silica β prepared by Takewaki et al. (10) had a water adsorption capacity of approximately 0.02 g/g at 298 K and a relative pressure of p/po ) 0.8. At a similar relative pressure, our all-silica β had a water adsorption capacity of 0.016 g/g at 293 K.
FIGURE 3. Water adsorption isotherms at 293 K on β zeolites. Isotherms were measured 3 times on all-silica β; the relative deviation of the adsorption capacity at higher pressure (p/po ≈ 0.9) was (3.0%.
FIGURE 5. MTBE adsorption isotherms at 298 K on all-silica β zeolite and on dealuminated mordenite zeolite reported by Anderson (5). Concentrations measured over 3 d; the percent recovery of the LCS each day was between 75.5 and 105%.
TABLE 2. Equilibration of 5 mg of β Zeolites in 25 mL of 11 000 µg of MTBE/L of Aqueous Solution β zeolite
MTBE equilibrium concn (µg/L)a
MTBE removed (%)
Hdealuminated all-silica
7,700 1,200 590b
30 89 95
a The initial solution (11 000 µg of MTBE/L) was analyzed at dilutions factors of 50 and 1000. The relative deviation of MTBE concentrations was (8.1%. The percent recovery of the LCS measured at the same time was 91%. b Sample was diluted by a factor of 10 and analyzed twice. The relative deviation was (0.34% (593 and 589 µg/L).
FIGURE 4. 2-Propanol adsorption isotherms at 293 K on β zeolites. Isotherms on all-silica β were measured twice; the relative deviation of the adsorption capacity at higher pressure (p/po ≈ 0.9) was (2.2%. In contrast to water adsorption, 2-propanol adsorption isotherms were less sensitive to the pressure, especially for the all-silica β (Figure 4). Even at low pressure, 2-propanol adsorption capacities for the three β zeolites were higher than 0.13 g/g. At high pressures, the amounts of 2-propanol and water adsorbed on H-β was almost the same. In contrast, the amounts of 2-propanol adsorbed on dealuminated β and all-silica β were approximately 1.5-9.0 and 7.0-77 times, respectively, the amounts of water adsorbed over the pressure range studied. That is, both dealuminated and all-silica β’s preferentially adsorb 2-propanol over water, but all-silica β is more hydrophobic. The H-β had similar micropore volume to the all-silica β (Table 1). However, it had a higher 2-propanol adsorption capacity than all-silica β, probably because it has a higher heat of adsorption. Bronsted acid sites (Si-OH-Al) in the H-β zeolite framework favor hydrocarbon adsorption. Dealuminated β had lower micropore volume than all-silica β (Table 1), but it had higher 2-propanol adsorption capacity at pressures p/po > 0.5. Mesoporous regions may have formed during dealumination (16), and they would favor capillary condensation of 2-propanol at high pressures. Stelzer et al. (11) used the competitive adsorption of vapor phase toluene/water or methylcyclohexane/water mixtures for their hydrophobicity index, which has been defined in other ways in the literature (17-19). We calculated a hydrophobicity index as the ratio of adsorbed 2-propanol
over water at 90% of saturated vapor pressure for each species. As shown in Table 2, the all-silica β has the highest hydrophobicity index. Thus, in aqueous organic mixtures, the all-silica β zeolite is expected to have the highest selectively for adsorbing organic molecules from water. MTBE Adsorption from Water. Table 2 shows the MTBE equilibrium concentration and percent of MTBE removed after equilibration of 25 mL of 11 000 µg of MTBE/L of aqueous solution with 5 mg of β zeolite. The all-silica β adsorbed 95% of the MTBE from water at an equilibrium MTBE concentration of 590 µg/L. This is expected since allsilica β is the most hydrophobic, and thus a MTBE adsorption isotherm was measured for it at 298 K. Figure 5 shows that the MTBE adsorption capacity increased linearly (R 2 of 0.997) with the MTBE equilibrium concentration, yielding a partition coefficient (slope of MTBE adsorption capacity vs MTBE equilibrium concentration) of 87 L/g. Within experimental accuracy, the dealuminated mordenite reported by Anderson exhibited the same behavior (5), but the highest MTBE equilibrium concentration he investigated was only about 120 µg/L. The partition coefficient for dealuminated mordenite was 80.1 L/g. The all-silica β powder was also used in a small packed column. A solution with 47 µg of MTBE/L passed downward through the column (50 mg of zeolite) at a flow rate of 3.0 mL/h. The MTBE concentration for the sample collected in the first 10 h was below the detection limit (2.5 µg/L). The all-silica β powder was regenerated by heating to 773 K for 4 h, although it is likely the MTBE could have been removed at lower temperature.
Acknowledgments We gratefully acknowledge support by the University of Colorado and MemPro Corporation. We also thank Dr. Bob VOL. 37, NO. 17, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Barkley in the Department of Chemistry and Biochemistry at University of Colorado for helpful discussions on MTBE concentration measurements.
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Received for review December 10, 2002. Revised manuscript received June 11, 2003. Accepted June 23, 2003. ES0264044
