Adsorption of pyridine on dealuminated zeolite HY H. -C. Wang a ; H. Paul Wang b ; Kuen-Song Lin b a

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1 This article was downloaded by: [National Cheng Kung University] On: 21 July 2010 Access details: Access Details: [subscription number ] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Journal of Environmental Science and Health, Part A Publication details, including instructions for authors and subscription information: Adsorption of pyridine on dealuminated zeolite HY H. -C. Wang a ; H. Paul Wang b ; Kuen-Song Lin b a Department of Internal Medicine, Veterans General Hospital, Tainan, Taiwan, R.O.C. b Department of Environmental Engineering, Cheng Kung University, Tainan, Taiwan, R.O.C. To cite this Article Wang, H. -C., Wang, H. Paul and Lin, Kuen-Song(1999) 'Adsorption of pyridine on dealuminated zeolite HY', Journal of Environmental Science and Health, Part A, 34: 9, To link to this Article: DOI: / URL: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 J. ENVIRON. SCI. HEALTH, A34(9), (1999) ADSORPTION OF PYRIDINE ON DEALUMINATED ZEOLITE HY Key Words: Dealumination, pyridine, VOCs, FTIR, adsorption H.-C. Wang 1, H. Paul Wang 2, * and Kuen-Song Lin 2 1 Department of Internal Medicine, Veterans General Hospital, Kaoshiung, 2 Department of Environmental Engineering, Cheng Kung University, Tainan, Taiwan, R.O.C. ABSTRACT Infrared spectra showed that over 80% of aluminum atoms on the surface of zeolite HY were extracted by a exposure of SiCl 4 vapor at elevated temperatures. The dealumination of the HY surface resulted in an appreciable change in its acidic properties. Infrared spectra of adsorbed pyridine indicated the formation of various kinds of Lewis acid sites on the delauminated HY surfaces. Although the total number of the surface Bronsted acid site decreased in the SiCl 4 -induced dealumination process, the nature of these remaining Brø nsted acid sites was unchanged spectroscopically. INTRODUCTION Zeolites are crystalline aluminosilicates containing an assemblage of SiO 4 and A1O 4 tetrahedron joined together by oxygen atoms to form a microporous solid, which has a precise pore structure (Ward, 1967). Because of the enhanced adsorption potential in micropores, zeolites are uniquely suitable as 1849 Copyright 1999 by Marcel Dekker, Inc.

3 1850 WANG, WANG, AND LIN adsorbents. Various forms of zeolites including zeolites A (3 À, 4Â, and 5Â), Faujasites (X and Y) and Pentasils (ZSM-5) have been used as commercial adsorbents (Ruthven, 1988). Activated carbons are commonly used to remove or recycle VOCs from air or water, but they require a relatively complex regeneration or disposal procedure. The carbon adsorption processes are not only expensive but also energy intensive. Moreover, VOCs are generally present in a humid stream where the adsorption of VOCs is negatively affected by the adsorption of water. A zeolite with a high silicon content would be more suitable for increasing in VOCs sorption and decreasing in water pickup. Zeolite Y is widely used in many commercial processes involving hydrocarbon synthesis, thermal and hydrothermal cracking, isomerization, and adsorption. Nonetheless, zeolite Y with a high silicon content (SiO^AljOj >10) may not be synthesized directly. The dealumination of a zeolite Y without destruction of the zeolite structure (by treating with SiCl 4 vapor) has been reported (Beyer et al., 1985; Anderson and Klinowski, 1986; Herrero, 1991; Boreave et al., 1997; Lutz et al., 1997; Abramova et al., 1998). However, the use of dealuminated zeolite adsorbents for the sorption of VOCs has been covered only to a limited extent in the literature (Prakash et al, 1995). Infrared spectroscopy has been frequently used in elucidating structure and bonding at solid surfaces (Wang et al., 1984; 1990; 1991). Because of its high resolution, infrared spectroscopy is also an excellent tool for measuring weak interactions in the adsorbed layer where small frequency shifts are observed. Thus, the main objective of the present work was to investigate the adsorption of pyridine on the acidic sites of the SiCl 4 -induced dealuminated zeolite HY by infrared spectroscopy.

4 ADSORPTION OF PYRIDINE ON ZEOLITE H Y 1851 EXPERIMENTAL Infrared spectra were recorded on a Digilab FT-IR spectrometer (FTS-40) with fully computerized data storage and data handling capability. For all spectra reported, a 64-scan data accumulation was carried out at a resolution of 4 cm. In order to observe small changes in band intensity, subtraction procedures were used to obtain difference spectra that more effectively compare spectra measured under various conditions. NE^t-form zeolite Y obtained from Strem Chemicals was calcined at 773 K for 16 hours to convert it to H-form zeolite (HY). About 50 mg of the zeolite HY sample (40-60 mesh) was placed in a controlled environmental chamber (Spectra Tech Inc.) designed for diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The sample was degassed at 673 K for one hour in flowing argon (30 ml/min). Infared spectra were measured during and after does of SiCU vapor that was directed onto HY. Infrared spectra of adsorbed pyridine saturatedly on the original and dealuminated zeolite HY were recorded at 300 K. RESULTS AND DISCUSSION Mid-infrared spectroscopy was used to monitor the state of hydroxyl groups on zeolite HY during the surface dealumination process. Figure 1 shows the difference in the infrared spectra of SiCl 4 -induced dealuminated HY at four elevated temperatures. Each spectrum represents the difference between an infrared spectrum obtained during the first ten-minute interval that SiCl 4 molecules were directed onto the zeolite surface and the original spectrum. The most notable feature of these difference spectra (Figure 1) was an absorbance decrease in the 3500 to 3800 cm frequency range characteristic of hydroxyl group absorption as the SiCl 4 molecules react. The 3732 cm feature has been

5 1852 WANG, WANG, AND LIN o WAVENUMBERS, cm" FIGURE 1 Infrared difference spectra measured between original and dealuminated HY by exposure to SiCL, vapor for 10 minutes at (a) 472 K, (b) 528 K, (c) 579 K, and (d) 620 K. assigned as the terminal non-acidic hydroxyl groups (SiOH) (Ward, 1970), and the 3708 cm feature as the hydroxyl nests (Anderson and Klinowski, 1986). The negative features at 3732 and 3708 cm (in Figure 1) indicated a low concentration of hydroxyl defect sites for the dealuminated HY. Ward (Ward, 1967) observed the appearance of infrared features associated with hydroxyl groups at 3640 and 3550 cm which were attributed to Brensted

6 ADSORPTION OF PYRIDINE ON ZEOLITE HY 1853 o in WAVENUMBERS, W 3300 FIGURE 2 Infrared spectra of HY dealuminated by exposure to SiCl 4 vapor at 620 K (a) before exposure to SiCl 4, (b) after 10 minutes of exposure, (c) after 20 minutes, and (d) after 30 minutes. The insert is explained in the text. acid sites. The composition of the structure containing the clusters of the OH-II (3640 cm ) or OH-II 1 (3550 cm ) type can be represented as (AlSi 2 O 3 )(SiOHAl)(O 3 Si 3 ). Figure 2 shows the infrared spectra of the original and SiCl 4 -induced dealuminated HY measured after each 10-minute dose of SiCl 4 molecules. Frequency shifts are not observed for all spectra measured. Typical isotherms for infrared absorbances changing with time at 3631 cm

7 1854 WANG, WANG, AND LIN 1XÍ U 5 3 o WAVENUMBERS, cm" 1 FIGURE 3 Différence spectrum measured between initial (pyridine preadsorbed) and dealuminated HY. B denotes Brensted acid sites and L denotes Lewis acid sites (OH-II), 3550 cm" 1 (OH-II 1 ), and 3734 cm" 1 (SiOH), respectively, are shown in the upper corner of Figure 2. Each point corresponds to the absorbance at each hydroxyl group taken from the infrared spectra in Figure 2. It was observed that the changes in degree of dealumination for OH-II and OH-II 1 showed a similar trend. It also appeared that the initial 10-minute SiCI 4 treatment is responsible for at least 80% of the surface dealumination.

8 ADSORPTION OF PYRIDINE ON ZEOLITE HY 1855 < WAVENUMBERS, cm"' FIGURE 4 Adsorption of pyridine on original and dealuminated HY. Pyridine adsorbed (a) on original HY, (b) on dealuminated HY (at 620 K), and (c) on dealuminated HY (at 579 K) It is known that dealumination of Y may lead to an appreciable change in its acidic properties (Anderson and Klinowski, 1986). However, the mechanism of the redistribution of the acid centers and the nature of the phenomena occurring during dealumination have not yet been sufficiently studied. It is also important to note the role played by Lewis acid sites during dealumination of HY. The

9 1856 WANG, WANG, AND LIN nature of the Lewis acid sites was studied by infrared spectroscopy by adsorption of pyridine. Pyridine bound to Lewis acid sites was associated with an infrared spectrum with characteristic absorption bands at 1460, 1490, and 1620 cm and that bound to Bransted acid sites with bands at 1490, 1545, and 1640 cm (Anderson and Klinowski, 1986). Figure 3 shows the difference infrared spectrum arising from a comparison of pyridine preadsorbed HY and the later dealuminated sample. The most significant result noted in the difference.i spectrum in Figure 3 is the very small change for Lewis acid sites (1460 cm ) during SiCl 4 -induced surface dealumination. This suggests that SiCl 4 vapor does not react with aluminum atoms in the Lewis acid sites. It is likely that the formation of various kinds of Lewis acid sites could be observable after the surface dealumination. Figure 4 shows that this was true in adsorption of pyridine on the original and dealuminated zeolite HY. It is obvious that "inhomogeneous broadening" was present for the infrared band at about 1460 cm which is associated with Lewis acid sites. Furthermore, the constancy at 1545 cm for both original and dealuminated HY suggests that the Brensted acid sites are chemically similar. CONCLUSIONS By using in situ diffuse reflectance infrared spectroscopy, the structural changes as well as surface perturbations that occur during surface dealumination of HY by SiCl 4 vapor were observed. The infrared difference spectra revealed that an aluminum atom was extracted under the dealumination conditions. An inhomogeneous broadening of the infrared feature at about 1460 cm which is associated with the formation of various kinds of Lewis acid sites on the dealuminated HY was also observed. Thus, the dealuminated HY would be very suitable for enhancement of the VOCs adsorption with minimized perturbation of water vapor.

10 ADSORPTION OF PYRIDINE ON ZEOLITE H Y 1857 ACKNOWLEDGEMENTS The financial support of the National Science Council of Taiwan (NSC E and NSC l-P ) are gratefully acknowledged. REFERENCES Abramova, A.V., Slivinskii, E.V. and Skryleva, E.A., Kinetics & Catal., 39, (1998). Anderson, M.W. and Klinowski, J., J. Chem. Soc.- Faraday Trans. I., 82, (1986). Anderson, M.W. and Klinowski, J., J. Chem. Soc.- Faraday Trans. I., 82, (1986). Anderson, M.W. and Klinowski, J., Zeolites, 6, (1986). Beyer, H.K., Belenykaja, I.M., Hange, F., Tielen, M., Grobet, P.J. and Jacobs, P.A., J. Chem. Soc.- Faraday Trans. I., 81, (1985). Boreave, A., Auroux, A. and Guimon, C., Microporous Materials, 11, (1997). Herrcro, C.P., J. Chem. Soc.- Faraday Trans. I., 87, (1991). Lutz, W, Loffler, E., Fechtelkord, M., Schreier, E. and Bertram, R., Studies in Surface Sci. & Catal., 105, (1997). Prakash, D.S., Athota, K.V. and Greene, H.L., AIChE Symp. Ser. 309, 91, 1-18 (1995). Ruthven, D.M., Chem. Eng. Prog., 84, (1988). Wang, H.P. and Yates, J.T., J. Catal., 89, (1984). Wang, H.P., Eyring, E.M. and Huai, H.A., Appl. Spect., 45, (1991). Wang, H.P., Yu, T., Garland, B.A., and Eyring, E.M., Appl. Spect., 44, (1990).

11 1858 WANG, WANG, AND LIN Ward, J.W., J. Catal., 9, (1967). Ward, J.W., J. Catal., 18, (1970). Received: January 27, 1999