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Synthesis and characterization of transition-metal-incorporated beta-zeolites
H.G. Karge and J. Weitkamp (Eds.) Zeolite Science 1994: Recent Progress and Discussions
Studies in Surface Science and Catalysis, Vol. 98 9 1995 Elsevier Science B.V. All rights reserved.
SYNTHESIS AND C H A R A C T E R I Z A T I O N OF TRANSITION-METALI N C O R P O R A T E D BETA-ZEOLITES Shu-Hua ChiCn*, Yung-Kuan Tseng, Maw-Chen Lin and Jen-Cheng Ho Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, ROC. SUMMARY. The incorporation of transition-metal ions in Beta-zeolite was carried out by direct hydrothermal synthesis. The synthesized zeolites (H-, Ti- and V-Beta) were characterized by powder X-ray diffraction (XRD), scanning electron microscopy with x-ray energy dispersive analyzer (SEM/EDS), infrared (IR), UV-visible and electron paramagnetic resonance (EPR) spectroscopies. Both XRD and IR spectroscopic studies confirmed that the synthesized Ti-Beta carried out isomorphous substitution of Si by Ti in the Beta-zeolite framework. In case of V-Beta, the EPR studies evidenced the formation of VO 2+ species that seem to be located at the cation sites of the zeolites. INTRODUCTION.
The composite metal oxide-zeolite materials have attracted
attention because of their application as bifunctional catalysts. Due to their reducibility and notable catalytic properties, the zeolites with incorporated titanium and vanadium are of particular interest. The aims of the present study are to synthesize large pore titanium- and vanadium- containing Beta-zeolites by direct hydrothermal method, and to well characterize the synthesized materials for the promising catalytic selective oxidation reactions. EXPERIMENTAL. The transition-metal containing Beta-zeolites (V-Beta and TiBeta) were synthesized by direct hydrothermal method, using tetraethyl ammonium hydroxide (TEA-OH),
amorphous Aerosil silica, aluminum nitrate, and
tetrabutylorthotitanate or vanadium oxide. The procedures were as follows: an aqueous solution of tetrabutylorthotitanate (or vanadium oxide) was oxidized by hydrogen peroxide first, then added with the aqueous solution of TEA-OH, Aerosil silica and finally aluminum nitrate. The mixture was stirred in a water-bath at 80oc for 30 minutes before transferring to an autoclave, which was then heated in an oven at 140oc for 20 days. After cooling the autoclave, the sample was centrifuged at 10000 rpm, the solid was calcined at 550oc. The synthesized zeolites were well characterized by XRD, SEM/EDS, IR, Uv-vis and EPR spectroscopies. The atomic ratios in the three samples are as follows: A1/Si = 1/30 in H-Beta zeolites, Ti/AI/Si = 1/1/30 in Ti-Beta and V/AI/Si = 2/1/30 in V-Beta. RESULTS AND DISCUSSION.
We have successfully synthesized the H-form
(H-Beta), Ti- and V- containing Beta (Ti-Beta and V-Beta) zeolites
by direct
hydrothermal method with A1/Si atomic ratio = 1/30. The powder x-ray diffraction
patterns of the three samples show good crystalline structures in Beta-form zeolites. The SEM micrographs of the three samples exhibit almost the same morphologies of cubic shapes. The average particle sizes are about 0.6 lam, no visible differences among the three. For the synthesized Ti-Beta zeolite, there appears isomorphous substitution of Si by Ti in the zeolite Beta framework, which is confirmed by the increase in the interplanar d-spacing in Ti-Beta as compared to bare H-Beta zeolite. Estimation was taken from the most intense peak at 20 -- 22.60 of the powder x-ray diffraction pattern following the method given in Ref. [1]. The intense IR band at 960 cm "l also gives the evidence of the successful substitution. Besides, an X-ray microprobe examination demonstrated that the titanium is uniformly distributed within the crystal. The EPR spectrum (taken at 77 K) of the evacuated sample exhibits an intense symmetric signal at g = 2.0030 due to the F-center and the signals at gl = 2.025, gz = 2.010 and g3 = 2.0024 due to the presence of 02- in the orthohombic geometry. The typical EPR signals of 02 - were remarkably enhanced when contacting with low pressure oxygen. We have tried to reduce the sample in hydrogen at high temperature, but no visible Ti 3+ EPR signals were observed, while traces of 02- still appeared. The 02- is known to be responsible for the selective oxidation reaction. In the case of the synthesized V-Beta zeolite, it is surprising that no vanadium signal was detected by EDS. The vanadium ions might hide inside the zeolites. The powder XRD pattern of V-Beta is very similar to the H-Beta zeolite and the interplanar d-spacing shows only a subtle increase. The infrared spectrum is similar to that of H-Beta, gives no evidence on the formation of V=O bond or Si-O-V species. However, evacuation of the V-Beta sample at 500oc, it appeared a distinguished EPR spectrum at g~= 1.930 and gi = 1.989 with A~ = 201.1 G and A• = 84.4 G rising from 51V (I = 7/2), which is most likely to be due to VO 2§ in the cation sites [2]. The high temperature interaction between V205 and the zeolite must have taken place during calcination. Apparently, the incorporation of titanium ions into the framework of Beta zeolite is successful in the present hydrothermal synthesis, while vanadium ions seem to prefer to occupy the cation sites.
The consequential
performance may be due to the different electronic properties of both ions. We believe that the results are important and helpful in the catalytic processes occurring in these systems. References: 1. M. Taramasso, G. Perego and B. Notari, US pat. 4 410 501, 1983. 2. M. Petras and B. Wichterlova, J. Phys. Chem. 1992, 96, 1805.
Studies in Surface Science and Catalysis, Vol. 98 9 1995 Elsevier Science B.V. All rights reserved.
SYNTHESIS AND C H A R A C T E R I Z A T I O N OF TRANSITION-METALI N C O R P O R A T E D BETA-ZEOLITES Shu-Hua ChiCn*, Yung-Kuan Tseng, Maw-Chen Lin and Jen-Cheng Ho Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, ROC. SUMMARY. The incorporation of transition-metal ions in Beta-zeolite was carried out by direct hydrothermal synthesis. The synthesized zeolites (H-, Ti- and V-Beta) were characterized by powder X-ray diffraction (XRD), scanning electron microscopy with x-ray energy dispersive analyzer (SEM/EDS), infrared (IR), UV-visible and electron paramagnetic resonance (EPR) spectroscopies. Both XRD and IR spectroscopic studies confirmed that the synthesized Ti-Beta carried out isomorphous substitution of Si by Ti in the Beta-zeolite framework. In case of V-Beta, the EPR studies evidenced the formation of VO 2+ species that seem to be located at the cation sites of the zeolites. INTRODUCTION.
The composite metal oxide-zeolite materials have attracted
attention because of their application as bifunctional catalysts. Due to their reducibility and notable catalytic properties, the zeolites with incorporated titanium and vanadium are of particular interest. The aims of the present study are to synthesize large pore titanium- and vanadium- containing Beta-zeolites by direct hydrothermal method, and to well characterize the synthesized materials for the promising catalytic selective oxidation reactions. EXPERIMENTAL. The transition-metal containing Beta-zeolites (V-Beta and TiBeta) were synthesized by direct hydrothermal method, using tetraethyl ammonium hydroxide (TEA-OH),
amorphous Aerosil silica, aluminum nitrate, and
tetrabutylorthotitanate or vanadium oxide. The procedures were as follows: an aqueous solution of tetrabutylorthotitanate (or vanadium oxide) was oxidized by hydrogen peroxide first, then added with the aqueous solution of TEA-OH, Aerosil silica and finally aluminum nitrate. The mixture was stirred in a water-bath at 80oc for 30 minutes before transferring to an autoclave, which was then heated in an oven at 140oc for 20 days. After cooling the autoclave, the sample was centrifuged at 10000 rpm, the solid was calcined at 550oc. The synthesized zeolites were well characterized by XRD, SEM/EDS, IR, Uv-vis and EPR spectroscopies. The atomic ratios in the three samples are as follows: A1/Si = 1/30 in H-Beta zeolites, Ti/AI/Si = 1/1/30 in Ti-Beta and V/AI/Si = 2/1/30 in V-Beta. RESULTS AND DISCUSSION.
We have successfully synthesized the H-form
(H-Beta), Ti- and V- containing Beta (Ti-Beta and V-Beta) zeolites
by direct
hydrothermal method with A1/Si atomic ratio = 1/30. The powder x-ray diffraction
patterns of the three samples show good crystalline structures in Beta-form zeolites. The SEM micrographs of the three samples exhibit almost the same morphologies of cubic shapes. The average particle sizes are about 0.6 lam, no visible differences among the three. For the synthesized Ti-Beta zeolite, there appears isomorphous substitution of Si by Ti in the zeolite Beta framework, which is confirmed by the increase in the interplanar d-spacing in Ti-Beta as compared to bare H-Beta zeolite. Estimation was taken from the most intense peak at 20 -- 22.60 of the powder x-ray diffraction pattern following the method given in Ref. [1]. The intense IR band at 960 cm "l also gives the evidence of the successful substitution. Besides, an X-ray microprobe examination demonstrated that the titanium is uniformly distributed within the crystal. The EPR spectrum (taken at 77 K) of the evacuated sample exhibits an intense symmetric signal at g = 2.0030 due to the F-center and the signals at gl = 2.025, gz = 2.010 and g3 = 2.0024 due to the presence of 02- in the orthohombic geometry. The typical EPR signals of 02 - were remarkably enhanced when contacting with low pressure oxygen. We have tried to reduce the sample in hydrogen at high temperature, but no visible Ti 3+ EPR signals were observed, while traces of 02- still appeared. The 02- is known to be responsible for the selective oxidation reaction. In the case of the synthesized V-Beta zeolite, it is surprising that no vanadium signal was detected by EDS. The vanadium ions might hide inside the zeolites. The powder XRD pattern of V-Beta is very similar to the H-Beta zeolite and the interplanar d-spacing shows only a subtle increase. The infrared spectrum is similar to that of H-Beta, gives no evidence on the formation of V=O bond or Si-O-V species. However, evacuation of the V-Beta sample at 500oc, it appeared a distinguished EPR spectrum at g~= 1.930 and gi = 1.989 with A~ = 201.1 G and A• = 84.4 G rising from 51V (I = 7/2), which is most likely to be due to VO 2§ in the cation sites [2]. The high temperature interaction between V205 and the zeolite must have taken place during calcination. Apparently, the incorporation of titanium ions into the framework of Beta zeolite is successful in the present hydrothermal synthesis, while vanadium ions seem to prefer to occupy the cation sites.
The consequential
performance may be due to the different electronic properties of both ions. We believe that the results are important and helpful in the catalytic processes occurring in these systems. References: 1. M. Taramasso, G. Perego and B. Notari, US pat. 4 410 501, 1983. 2. M. Petras and B. Wichterlova, J. Phys. Chem. 1992, 96, 1805.