Energy & Fuels 1998, 12, 1051-1054
1051
VOC Removal: Comparison of MCM-41 with Hydrophobic Zeolites and Activated Carbon X. S. Zhao, Q. Ma, and G. Q. (Max) Lu* Department of Chemical Engineering, University of Queensland, St Lucia, QLD 4072, Australia Received May 5, 1998. Revised Manuscript Received July 21, 1998
The recently discovered mesoporous molecular sieve MCM-41 was tested as an adsorbent for VOC removal. Its adsorption/desorption properties were evaluated and compared with other hydrophobic zeolites (silicalite-1 and zeolite Y) and a commercial activated carbon, BPL. The adsorption isotherms of some typical VOCs (benzene, carbon tetrachloride, and n-hexane) on MCM-41 are of type IV according to the IUPAC classification, drastically different from the other microporous adsorbents, indicating that VOCs, in the gas phase, have to be at high partial pressures in order to make the most of the new mesoporous material as an adsorbent for VOC removal. However, a proper modification of the pore openings of MCM-41 can change the isotherm types from type IV to type I without remarkable loss of the accessible pore volumes and, therefore, significantly enhance the adsorption performance at low partial pressures. Adsorption isotherms of water on these adsorbents are all of type V, demonstrating that they possess a similar hydrophobicity. Desorption of VOCs from MCM-41 could be achieved at lower temperatures (5060 °C), while this had to be conducted at higher temperatures (100-120 °C) for microporous adsorbents, zeolites, and activated carbons.
Introduction One of the most formidable challenges posed by the increasingly stringent regulations on air pollution in many countries is the search for efficient and economical control strategies for volatile organic compounds (VOCs). The most currently applicable technology for VOC control is adsorption on activated carbon with subsequent solvent recovery or incineration.1,2 However, it has been recognized that an activated carbon presents several disadvantages,3 such as fire risk, pore clog (due to polymerization of some VOCs catalyzed by ashes present on activated carbon surfaces), hygroscopicity, and some problems associated with regeneration, etc. Hence, much effort has been focused at finding alternative adsorbents. Hydrophobic zeolite adsorbents have been proven to be an advancement in VOC adsorption/ separation technology.4 By employing a hydrophobic zeolite or molecular sieve, the problems associated with activated carbon adsorbents could be overcome. In 1992, a novel mesoporous molecular sieve family, M41S, was discovered by scientists at the Mobil Corp.5 MCM-41, one member of M41S family, possesses a high surface area and a large pore volume with highly ordered hexagonally packed cylindrical pores. Most importantly, both the surface chemistry and pore openings of MCM-41 can be tailored by postsynthesis to meet * To whom correspondence should be addressed. Fax: 61-733654199. E-mail:
[email protected]. (1) Rubby, E. N.; Carroll, L. A. Chem. Eng. Prog. 1993, 28-35. (2) Stenzel, M. H. Chem. Eng. Prog. 1993, 36-43. (3) Fajula, F.; Plee D. Stud. Surf. Sci. Catal. 1994, 85, 633-651. (4) Blocki, S. W. Environ. Prog. 1993, 12, 226-230. (5) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712.
a given requirement.6,7 A number of applications of this novel material have been suggested in catalysis,8 adsorption,9 and other relevant areas.10 In our group, we have proposed to develop MCM-41-based adsorbents/ catalysts for VOC emission control. In a practical VOC removal process, an ideal adsorbent is expected to have (1) a large amount of reversible adsorption capacity (large accessible pore volume), (2) no catalytic activity, (3) a hydrophobic property, (4) high thermal and hydrothermal stability, and (5) an easy regeneration property. An adsorption isotherm can provide much information concerning the saturated adsorption capacity, the surface property, and the capillary condensation point (if exists) of an adsorbent. On the other hand, a desorption curve can tell one the regeneration temperature, reversible or irreversible adsorption, and the desorption enthalpy and entropy of an adsorbent. In the present paper, the adsorptiondesorption properties of MCM-41 were evaluated and compared with some of the other hydrophobic zeolites, i.e., dealuminated zeolite Y and silicalite-1, and a commercial activated carbon BPL in terms of the adsorption isotherm and temperature-programmed desorption (TPD) curve. Experimental Section Adsorbent Samples. The MCM-41 sample was prepared using dodecyltrimethylammonium bromide as the template. (6) Zhao, X. S.; Lu, G. Q. J. Phys. Chem.B 1998, 102, 1556-1561. (7) Zhao, X. S.; Lu, G. Q. Nature 1998, manuscript in preparation. (8) Corma, A. Chem. Rev. 1997, 97, 2373-2419. (9) Beck, J. S.; Vartuli, J. C. Curr. Opin. Solid State Mater. Sci. 1996, 1, 76-87. (10) Zhao, X. S.; Lu, G. Q.; Millar, G. J. Ind. Eng. Chem. Res. 1996, 35, 2075-2082.
10.1021/ef980113s CCC: $15.00 © 1998 American Chemical Society Published on Web 09/04/1998
1052 Energy & Fuels, Vol. 12, No. 6, 1998
Zhao et al.
Table 1. Characterization Results of the Four Adsorbents
XRD
BET surface area (m2/g)
total pore volume (mL/g)
BJH pore size distribution (PSD)
pore configuration
Si-MCM-41
d100 ) 32.3 Å
1060
0.87
very narrow PSD centered at 22.5 Å
hydrophobic zeolite Y silicalite-1 activated carbon BPL
FAU MFI N/A
692 371 923
0.31 0.21 0.46
very narrow PSD centered at 7.4 Å very narrow PSD centered at 5.5 Å bidispersed PSD 8.5 Å, 14.1 Å
1-D cylindrical pore, hexagonally packed 3-D cage-like pore 3-D channel-like pore Slit-shaped mainly, containing micropores
sample
The detailed synthesis procedures can be found elsewhere.11 Hydrophobic zeolite Y (Si/Al ) 300) was obtained by steaming and acid treatments of the parent NaY (Si/Al ) 2.45, from CU Chemie Uetikon AG, The Netherlands) according to the methods described by Ward12 and Fleisch et al.13 Silicalite-1 (pure-silica ZSM-5) was synthesized using tetrabutylammonium bromide as the template at 175 °C for 24 h. All zeolites and molecular sieves were calcined at 550 °C for 6 h before use. The commercial activated carbon, BPL, was obtained from Calgon without further treatment. Characterization. X-ray diffraction (XRD) patterns were recorded on a PW 1840 diffractometer with Co KR radiation. Nitrogen adsorption measurements were conducted at liquid nitrogen temperature using a NOVA 1200 analyzer (Quantachrome). Samples were outgassed at 553 K overnight before measurement. Surface areas were calculated using the BET model. Total pore volumes were estimated at a relative pressure of 0.95, assuming full surface saturation of nitrogen. Pore size distributions were obtained using the BJH model. Adsorption isotherms of three typical VOCs, i.e., benzene, carbon tetrachloride, and n-hexane, and water vapor were measured using a gravimetric technique using a quartz spring balance (Wilmad Glass). The sensitivity of the weight measurement is 0.01 mg. Before measurement, a sample was outgassed at 473 K overnight. The residual pressure within the adsorption chapter was below 5 × 10-4 Torr. The vapor pressure for an adsorptive was measured using the capacitance manometer (MKS Baratron 122 B) with a sensitivity of 0.1 Torr. Lower vapor pressures were also monitored using a silicon oil manometer, which were in excellent agreement with that measured by the transducer. The adsorption equilibrium for VOCs could be reached within 20 min, while the adsorption equilibrium for water was normally achieved within 60 min. Desorption of the organics was evaluated by temperatureprogrammed desoprtion (TPD) on a thermogravimetric analyzer (TGA) (Shimadzu). A sample was first dried at 150 °C overnight before being rapidly transferred into a desiccator containing the specific VOCs. Adsorption was conducted at room temperature (22 °C) and ambient pressure for 48-1200 h until no weight gain was observed by an analytical balance. Then a VOC-loaded sample was rapidly loaded into the TGA sample holder, and TPD was started in a pure helium atmosphere with a flow rate of 25 mL/min. The desorption curves were plotted as the first derivative TG (DTG).
Results and Discussion Sample Characterization. Shown in Table 1 are the characterization results of the various adsorbents. XRD results indicate that both the MCM-41 and silicalite-1 samples prepared in this study are highly qualified, as well as the dealuminated zeolite Y. The strong (100) diffraction together with the other three diffraction peaks at higher angles in the XRD pattern (11) Zhao, X. S.; Lu, G. Q.; Millar, G. J.; Li, X. S. Catal. Lett. 1996, 38, 33-37. (12) Ward, J. W. J. Catal. 1970, 18, 348-351. (13) Fleish, T. H.; Meyers, B. L.; Ray, G. J.; Hall, J. B.; Marshall, C. L. J. Catal. 1986, 99, 117. (14) Richards, R. E.; Rees, L. V. C. Zeolites 1986, 6, 17.
Figure 1. Adsorption isotherms of benzene over the various adsorbents.
demonstrate a good MCM-41 sample. The silicalite-1 sample shows a XRD pattern similar with that of ZSM5, characteristic of a MFI pore structure. Nitrogen adsorption measurements further confirmed the XRD results. All samples had reasonable pore volumes and surface areas. It can also be seen that the BET surface area of the activated carbon adsorbent is similar to that of MCM-41, whereas its pore volume is much lower than that of MCM-41. The pore volumes of hydrophobic zeolite Y and silicalite-1 are far lower compared to MCM-41. It is, therefore, expected that MCM-41would be a promising adsorbent for VOC removal. In addition, MCM-41 is a mesoporous material that is capable of encapsulating larger organic molecules with little diffusion resistance. Adsorption Isotherm. An adsorption isotherm can provide information on the adsorption capacity, hydrophobicity/hydrophilicity, and the capillary condensation point (if present) of an adsorbent. Figures 1-3 show the adsorption isotherms of benzene, carbon tetrachloride, and n-hexane over the various adsorbents at 22 °C, respectively. For the MCM-41 sample, adsorption isotherms of the organics are all typical type IV isotherms according to IUPAC classification. A monolayer-multilayer adsorption occurred before capillary condensation. The P/P0 points where capillary condensation occurred are centered at 0.15-0.2 for all three organics. For both the hydrophobic zeolites and activated carbon adsorbents, the adsorption isotherms are of a Langmuir type. The adsorption capacities of benzene and n-hexane on the adsorbents at lower concentration levels followed the sequence of activated carbon > hydrohpobic zeolite Y > silicalite-1 > MCM41, while at higher concentration levels, the sequence changed to MCM-41 > activated carbon > hydrophobic zeolite Y > silicalite-1. The saturated adsorption amounts of the adsorbents are in good agreement with
VOC Removal
Figure 2. Adsorption isotherms of carbon tetrachloride over the various adsorbents.
Energy & Fuels, Vol. 12, No. 6, 1998 1053
Figure 4. Adsorption isotherms of water over the various adsorbents.
Figure 5. Adsorption isotherms of benzene over MCM-41 samples before and after modification. Figure 3. Adsorption isotherms of n-hexane over the various adsorbents.
the results of nitrogen adsorption, demonstrating that all the pores are highly accessible to the organics except the pores of silicalite-1, which adsorbed little carbon tetrachloride. This is because carbon tetrachloride could not penetrate into the internal pores of silicalite-1 due to the smaller pores of silicalite-1 compared to the kinetic diameter of carbon tetrachloride (ca. 6.0 Å). These results indicate that MCM-41 does have large accessible internal pore volumes which can be filled at higher relative pressures compared to the microporous adsorbents. However, the type IV isotherm behavior of MCM-41 requires VOCs, in the gas phase, to be at a high partial pressure in order to fully fill the accessible pores of MCM-41. This is not the situation with most industrial applications where VOCs are normally present at low partial pressures. However, by properly tailoring the pore openings of MCM-41,7 the adsorption isotherm type can be modified from type IV to type I without much loss of the accessible pore volumes (see below). Shown in Figure 4 are the adsorption isotherms of water vapor on the adsorbents. We were interested in measuring water adsorption because the VOC stream contains a large amount of water. As expected, dealuminated zeolite Y and silicalite-1 are very hydrophobic without much loading of water. The surface hydropho-
bicity of MCM-41 is similar to that of activated carbon, reflected by their adsorption amounts at lower relative pressures. However, close inspection of the adsorption isotherms reveals that the P/P0 where capillary condensation occurred is larger for MCM-41 (0.55-0.60) than for the activated carbon PBL (0.40-0.45) (Figure 4). This difference is of significant importance since VOCs are generally present in VOC streams with a relative humidity of around 50%. Pore-Opening Modification. Figure 5 shows the adsorption isotherms of benzene over MCM-41 before and after pore modification, together with the isotherms of the activated carbon and hydrophobic zeolite Y for comparison. It is seen that after modification of the pore openings of MCM-41, the adsorption capacity of benzene at lower partial pressure was significantly enhanced without too much loss of the pore volumes. The adsorption capacity of the modified MCM-41 adsorbent is still higher than that of the microporous adsorbents, activated carbon, and hydrophobic zeolite Y. The saturated adsorption amount of benzene was decreased by about 25% after modification due to the partial loss of the pore volumes during modification. The adsorption type changed from type IV to type I as a result of the reduction of the pore openings of the modified MCM-
1054 Energy & Fuels, Vol. 12, No. 6, 1998
Figure 6. TPD profiles of benzene from the various adsorbents.
Zhao et al.
Figure 8. TPD profiles of n-hexane from the various adsorbents.
ing rates from 10/min to 2 °C/min, while this was not the case for MCM-41. These results demonstrate that VOCs are easier to be desorbed from the mesoporous adsorbent, MCM-41, than from the microporous adsorbents, hydrophobic zeolites, and activated carbons. This is very important from the consideration of energy saving since the VOC-loaded adsorbents are normally regenerated by hot steam in order to recover the solvent. Conclusion
Figure 7. TPD profiles of carbon tetrachloride from the various adsorbents.
41. More information concerning the modified MCM41 adsorbent is being obtained and will be reported elsewhere.7 TPD Study. Figures 6-8 show the DTG curves of benzene, carbon tetrachloride, and n-hexane from the four adsorbents. It can be seen that the maximum mass loss rates of all the organics from MCM-41 occurred at about 60 °C. In addition, the desorption peaks are very sharp, demonstrating that the desorption of organics from MCM-41 was very fast. However, the maximum mass loss rates from both the hydrophobic zeolites and activated carbon adsorbent were all found to occur above 100 °C. Also, the desorption peaks are broad, indicating that desorption of VOCs from these microporous adsorbents are slow. Two desorption peaks could be resolved on the microporous adsorbents by decreasing the heat-
It is concluded that hydrophobic MCM-41 is a potential adsorbent for the removal of VOCs present in high concentrations and high humidity streams. Proper modification of the pore openings of MCM-41 can significantly enhance the adsorption performance without a remarkable loss of the accessible pores. Hydrophobic zeolites are candidate adsorbents in removing VOCs in low concentrations with high humidity. A composite adsorbent composed of hydrophobic microporous zeolites and mesoporous molecular sieves, such as hydrophobic zeolite Y plus MCM-41, may be expected to be promising in VOC removal because it could handle VOCs in a larger spectrum of concentrations with high humidity. At the moment, activated carbon adsorbents will still be the first choice for use in air-flow treatment systems because they are costeffective materials, although some of the above-mentioned disadvantages are frequently encountered in practice. However, MCM-41 will be a particularly efficient and competitive adsorbent for VOC recovery at high concentrations or for applications where lowtemperature waste heat is available for regeneration. EF980113S
