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Synthesis and characterization of V-Beta zeolite
Zeolites: A Refined Tool for Designing CatalyticSites L. Bonneviot and S. Kaliaguine(editors) 9 1995 Elsevier Science B.V. All rights reserved.
533
Synthesis and Characterization of V - B e t a Zeolite Shu-Hua Chien and Jang-Cheng Ho Institute of Chemistry, Academia Sinica, Taipei 11529, and Department of Chemistry, National Taiwan University, Taipei 107, Taiwan, ROC
SUMMARY H-form and vanadium-containing Beta-zeolite (V-Beta) were synthesized by the hydrothermal method. XRD and IR spectra confirm that the structure of V-Beta is isomorphous with Beta zeolite. EPR spectra indicate VO 2§ ions formed in both V-Beta and solid mixture of V20 5 and H-Beta zeolite and most likely located at cationic sites. The 29Si MAS-NMR spectra indicate that Si-O-A1 angle of V-Beta is larger than that of H-Beta. Infrared spectra of pyridine adsorption show both H-Beta and V-Beta possess highly Lewis acid property but more acid sites in the V-Beta. INTRODUCTION The localization of vanadium ions in synthesized zeolites has gained particular interest owing to their bifunctional and notable catalytic properties. In this paper, we prepared V-Beta zeolite by the direct hydrothermal method and characterized well the structure of the synthesized material. For comparison, the solid mixture of vanadium oxide and H-beta zeolite was also investigated. EXPERIMENTAL The H- and V- Beta zeolites were hydrothermally synthesized using aluminum nitrate, amorphous Aerosil silica and tetraethylammonium hydroxide (TEA-OH) as starting materials. In the preparation of V-Beta, the V205 was used and preoxidized by H20 2. The reactant mixtures were placed in a teflon-lined autoclave and heated at 140~ for 20 days. The resulting sample was centrifuged and washed, then dried at 120~ Calcination was carried out in 0 2 stream at 550~ for 15 hours. The mixture of V20 5 and H-Beta with 0.3 mmol V per 1 g H-Beta was ground mechanically in a vibration mill. The synthesized zeolites were well characterized by ICP-AES, TGA, XRD, SEM-EDS, FT-IR, UV-VIS, 29Si and 27A1 MAS-NMR and EPR spectroscopy.
534 RESULTS AND DISCUSSION
We have successfully synthesized H-Beta and V-Beta zeolites by direct hydrothermal method with Si/A1 atomic ratio of about 25/1. ICP-AES measurement gives a V content of 0.047 wt% in V-Beta, but no vanadium signal was detected by EDS. TGA profiles show the organic materials were removed below 600~ in both as-synthesized zeolites. The XRD patterns of both zeolites are highly crystalline and match very closely with the Beta zeolite structure. The SEM micrographs exhibit almost the same morphologies in cubic shape. The average crystal size of H-Beta is about 0.5 larn and the V-Beta is slightly larger. FT-IR spectra exhibit the characteristic peaks of Beta zeolite but the peaks at 1098 and 1230 cm 1 of H-Beta shift to 1103 and 1237 cm -1 and the peak at 953 cm 1 almost disappeared in V-Beta. UV-VIS spectrum of H-Beta gives a weak peak at 222 nm. In V-Beta, a very broad absorption band appeared from 200 nm up to 370 nm with overlapping peaks likely appearing at 222, 248, 287 and 330 nm. Both FT-IR and UV-VIS spectra reveal that the incorporation of vanadium indeed affects the structure of H-Beta. EPR spectra of V-Beta at 77 K showed intense signals at g//= 1.936 and ga_ = 1.987 with A// = 201 G and At_ = 84 G, with distinct eight-lines hyperfine splittings due to 51V (I = 7/2). After the sample being evacuated at 298 K, the spectrum retained almost the same g-values and hyperfine constants, but a super hyperf'lne structure with A = 4 G appeared. The mixture of V205 and H-Beta also gave the eight-lines hyperfine splitting spectra at g / / = 1.931 and g.t. = 1.985 with A// = 199 G and AJ_ = 81 G, but no super hyperfine structure was observed. The results are comparable to those results as VO 2+ adsorbed on ZSM-5 and X, Y zeolite [ 1,2]. Therefore, we believe that the VO 2+ species were formed in V-Beta zeolite and most likely located at cationic sites. The 29Si MAS-NMR spectrum of H-Beta showed a strong peak centered at -110.2 with a weak shoulder at-104.1 ppm. The spectrum of V-Beta also showed a strong but much narrower peak centered at -110.7 ppm, besides, a distinct small sharp peak appearing at -114.4 ppm. The Si/A1 ratio calculated from 29Si NMR using Loewenstein rule is 24.5 for H-Beta and 235 for V-Beta. Probably, these results are due to the increase in SiO-A1 angle [3] when VO 2+ occupy the cation sites. In the case of the 27A1 NMR spectrum, the resonance peak corresponding to tetrahedral A1 is composed of two overlapping peaks at 56.5-ppm and 54.2-ppm. For H-Beta, the peak intensity of 56.5-ppm is slightly larger. On the other hand, the intensity of 56.5-ppm peak of V-Beta decreased and became to about half of the 54.2-ppm peak. Both H-Beta and V-Beta exhibited a signal at 0.1 ppm corresponding to octahedral A1, but a broader peak with a visible shoulder appeared at higher ppm for V-Beta. Moreover, the infrared spectra of pyridine adsorbed on both H-Beta and V-Beta zeolites indicate that both zeolites possess highly Lewis acidic and very weak BrCnsted acidic properties with more acid sites in the V-Beta. We believe that the results are important and helpful in the related catalytic processes. REFERENCES
1. M. Petras and B. Wichterlova, J. Phys. Chem., 96 (1992) 1805. 2. G. Martini, M.F. Ottaviani and G.L. SeravaUi, J. Phys. Chem., 79 (1975) 1716. 3. J.M. Thomas, J. Klinowski, S. Ramdas, B.K. Hunter and D.T.B. Tennakoon, Chem. Phys. Lett., 102 (1983) 158.
533
Synthesis and Characterization of V - B e t a Zeolite Shu-Hua Chien and Jang-Cheng Ho Institute of Chemistry, Academia Sinica, Taipei 11529, and Department of Chemistry, National Taiwan University, Taipei 107, Taiwan, ROC
SUMMARY H-form and vanadium-containing Beta-zeolite (V-Beta) were synthesized by the hydrothermal method. XRD and IR spectra confirm that the structure of V-Beta is isomorphous with Beta zeolite. EPR spectra indicate VO 2§ ions formed in both V-Beta and solid mixture of V20 5 and H-Beta zeolite and most likely located at cationic sites. The 29Si MAS-NMR spectra indicate that Si-O-A1 angle of V-Beta is larger than that of H-Beta. Infrared spectra of pyridine adsorption show both H-Beta and V-Beta possess highly Lewis acid property but more acid sites in the V-Beta. INTRODUCTION The localization of vanadium ions in synthesized zeolites has gained particular interest owing to their bifunctional and notable catalytic properties. In this paper, we prepared V-Beta zeolite by the direct hydrothermal method and characterized well the structure of the synthesized material. For comparison, the solid mixture of vanadium oxide and H-beta zeolite was also investigated. EXPERIMENTAL The H- and V- Beta zeolites were hydrothermally synthesized using aluminum nitrate, amorphous Aerosil silica and tetraethylammonium hydroxide (TEA-OH) as starting materials. In the preparation of V-Beta, the V205 was used and preoxidized by H20 2. The reactant mixtures were placed in a teflon-lined autoclave and heated at 140~ for 20 days. The resulting sample was centrifuged and washed, then dried at 120~ Calcination was carried out in 0 2 stream at 550~ for 15 hours. The mixture of V20 5 and H-Beta with 0.3 mmol V per 1 g H-Beta was ground mechanically in a vibration mill. The synthesized zeolites were well characterized by ICP-AES, TGA, XRD, SEM-EDS, FT-IR, UV-VIS, 29Si and 27A1 MAS-NMR and EPR spectroscopy.
534 RESULTS AND DISCUSSION
We have successfully synthesized H-Beta and V-Beta zeolites by direct hydrothermal method with Si/A1 atomic ratio of about 25/1. ICP-AES measurement gives a V content of 0.047 wt% in V-Beta, but no vanadium signal was detected by EDS. TGA profiles show the organic materials were removed below 600~ in both as-synthesized zeolites. The XRD patterns of both zeolites are highly crystalline and match very closely with the Beta zeolite structure. The SEM micrographs exhibit almost the same morphologies in cubic shape. The average crystal size of H-Beta is about 0.5 larn and the V-Beta is slightly larger. FT-IR spectra exhibit the characteristic peaks of Beta zeolite but the peaks at 1098 and 1230 cm 1 of H-Beta shift to 1103 and 1237 cm -1 and the peak at 953 cm 1 almost disappeared in V-Beta. UV-VIS spectrum of H-Beta gives a weak peak at 222 nm. In V-Beta, a very broad absorption band appeared from 200 nm up to 370 nm with overlapping peaks likely appearing at 222, 248, 287 and 330 nm. Both FT-IR and UV-VIS spectra reveal that the incorporation of vanadium indeed affects the structure of H-Beta. EPR spectra of V-Beta at 77 K showed intense signals at g//= 1.936 and ga_ = 1.987 with A// = 201 G and At_ = 84 G, with distinct eight-lines hyperfine splittings due to 51V (I = 7/2). After the sample being evacuated at 298 K, the spectrum retained almost the same g-values and hyperfine constants, but a super hyperf'lne structure with A = 4 G appeared. The mixture of V205 and H-Beta also gave the eight-lines hyperfine splitting spectra at g / / = 1.931 and g.t. = 1.985 with A// = 199 G and AJ_ = 81 G, but no super hyperfine structure was observed. The results are comparable to those results as VO 2+ adsorbed on ZSM-5 and X, Y zeolite [ 1,2]. Therefore, we believe that the VO 2+ species were formed in V-Beta zeolite and most likely located at cationic sites. The 29Si MAS-NMR spectrum of H-Beta showed a strong peak centered at -110.2 with a weak shoulder at-104.1 ppm. The spectrum of V-Beta also showed a strong but much narrower peak centered at -110.7 ppm, besides, a distinct small sharp peak appearing at -114.4 ppm. The Si/A1 ratio calculated from 29Si NMR using Loewenstein rule is 24.5 for H-Beta and 235 for V-Beta. Probably, these results are due to the increase in SiO-A1 angle [3] when VO 2+ occupy the cation sites. In the case of the 27A1 NMR spectrum, the resonance peak corresponding to tetrahedral A1 is composed of two overlapping peaks at 56.5-ppm and 54.2-ppm. For H-Beta, the peak intensity of 56.5-ppm is slightly larger. On the other hand, the intensity of 56.5-ppm peak of V-Beta decreased and became to about half of the 54.2-ppm peak. Both H-Beta and V-Beta exhibited a signal at 0.1 ppm corresponding to octahedral A1, but a broader peak with a visible shoulder appeared at higher ppm for V-Beta. Moreover, the infrared spectra of pyridine adsorbed on both H-Beta and V-Beta zeolites indicate that both zeolites possess highly Lewis acidic and very weak BrCnsted acidic properties with more acid sites in the V-Beta. We believe that the results are important and helpful in the related catalytic processes. REFERENCES
1. M. Petras and B. Wichterlova, J. Phys. Chem., 96 (1992) 1805. 2. G. Martini, M.F. Ottaviani and G.L. SeravaUi, J. Phys. Chem., 79 (1975) 1716. 3. J.M. Thomas, J. Klinowski, S. Ramdas, B.K. Hunter and D.T.B. Tennakoon, Chem. Phys. Lett., 102 (1983) 158.