TiO2-ZrO2에 담지된 V2O5촉매의 특성과 산촉매 활성

박은희, 이만호, 손종락, 김해원*

경북대학교 공업화학과, *경일대학교 공업화학과

Characterization of V2O5 Supported on TiO2-ZrO2 and  Catalytic Activity for Acid Catalysis

Eun Hee Park, Man Ho Lee. Jong Rack Sohn, Hae Won Kim*

Dept. of Industrial Chemistry, Kyungpook National University, *Dept. of Industrial chemistry, Kyungil University

Introduction

Supported vanadium oxides are being used extensively as catalysts for a variety of reactions including partial oxidations and ammoxidations.1-3  Much research has been done to understand the nature of active sites, the surface structure of catalysts as well as the role played by the promoter of the supported catalysts, using Infrared(IR), 51V solid state NMR, X-ray diffraction(XRD), Electron spin resonance(ESR) and Raman spectroscopy.  The promoting effect of a TiO2 support on the oxidation of o-xylene on V2O5 has been absorbed to an increase of the number of surface V=O bond on the V2O5/TiO2 catalysts and weakening of thee bonds.4,5  51V solid state NMR has been used by varian investigators to differentiate between different kinds of vanadia species in the catalysts6.  In the present investigation, the techniques of 51V solid state NMR, X-ray diffraction, Fourier transform infrared(FTIR) and DSC have been utilized to characterize a series of V2O5 supported on TiO2-ZrO2 with various vanadia loadings.

Experimental

Catalyst preparation: Coprecipitate of Ti(OH)4-Zr(OH)4 was obtained by adding aqueous ammonia slowly into a mixed aqueous solution of titanium tetrachloride and zirconium oxychloride at room temperature with stirring until the pH of mother liquor reached about 8.  Ti(OH)4-Zr(OH)4 mixed hydroxide was synthesized with a 1:1 molar ratio.  The coprecipitate thus chloride ion was not detected by AgNO3 solution and was dried at 110℃ for 12 h.  The dried coprecipitate was powdered below 100 mesh by grinding.  The catalysis containing various vanadium oxide contents were prepared by adding an aqueous solution of ammonium metavanadate(NH4VO3) to the Ti(OH)4-Zr(OH)4 powder followed by ddring and calcining at high temperatures for 3 h in air.  This series of catalysts are denoted by their weight percentage of V2O5 and calcination temperature.

Characterization: FTIR absorption spectra of V2O5/TiO2-ZrO2 powders were measured by the KBr disk method over the range 1200-400 cm-1.  FTIR spectra of ammonia adsorbed on the catalyst were obtained in a heatable gas cell at room temperature using a Mattson Model GL 6030E spectrophotometer.  51V NMR spectra were measured by a Varian Unity Inova 300 spectrometer with a static magnetic field strength of 7.05 T.  Larmor frequency was 78.89 MHz.  The ordinary single pulse sequence was used, in which the pulse width was set at 2.8 s  and the acquisition time was 0.04 s.  The spectra width was 500 kHz.  The number of scans was varied from 400 to 4000, depending on the concentration of vanadium.  The sample was static, and its temperature was ambient.

Catalysts were checked in order to determine the structure by means of a Jeol Model JDX-8030 diffractometer, employing Cu□□(Ni filtered) radiation, DSC measurement were performed by a PL-STA model 1500H apparatus in air, and the heating rate was 5K per minute. For each experiment 10-15 mg of sample was used.  The specific surface area was determined by applying the BET method to the adsorption of N2 at 77 K.

Results and Discussion

Solid state NMR methods represent a novel and promising approach to vanadium oxide catalytic materials.  The 51V solid state NMR spectra of V2O5/TiO2-ZrO2 catalysts calcined at 773 K are shown in Fig.1.  There are three types of signals in the spectra of catalysts with varying intensities depending on V2O5 a shoulder at about 260 ppm and the intense peak -522 ∼ -666 ppm are observed.  The former is assigned to the surface vanadium-oxygen structures surrounded by a distorted octahedron of oxygen atoms, while the latter is attributed to   the tetrahedral vanadium-oxygen structure.  However, the surface vanadium oxide structure is remarkably dependent on the metal oxide support material.  Vanadium oxide on TiO2(anatase) displays the highest tendency to be 6-coordinated at low surface coverages, while in the case of □-Al2O3 a tetrahedral surface vanadium species is the favored.  as shown in Fig.1, at low vanadium loading on TiO2-ZrO2 a tetrahedral vanadium species is exclusively dominant compared with a octahedra coordination of vanadium oxide, while at higher surface coverages vanadium oxide becomes increasingly octahedral-coordinated.  Evacuation of the samples at 250℃ and 10-5 Torr for 2 h results in a change in the spectra(Fig.2).  For the

-344     -666

-551     25% V2O5

-540

10% V2O5

5% V2O5

0   -500  -1000  -1500

Fig.1 51V solid state NMR of V2O5/TiO2-ZrO2 samples as a function of

vanadia concentration

-660

-607

25% V2O5

-522      10% V2O5

5% V2O5

500    0    -500  -1000  -1500

Fig.2 51V solid state NMR of V2O5/TiO2-ZrO2 samples as a function of

vanadia concentration for evacuation of 250℃, 2h, 10-5Torr

evacuated samples of the position of the peak is shifted compared to nonevacuated samples.  They can be attributed to surface tetrahedral vanadia complexes which do not contain water molecules or OH groups in their first coordination sphere, since evacuation does not change the position of their peaks.  Fig.1 and Fig.2 can be attributed to surface tetrahedral complexes containing one or more OH groups or water molecules in their coordination sphere.  This is indicated by the change of the peak position for the Fig.1 after evacuation of the sample.  The decrease of the intensity of Fig.2 on evacuation compared to that of Fig.1, indicates that dehydration of 250℃ results in transformation of the main part of surface tetrahedral complexes to V2O5. Note that the surface species observed for the V2O5/TiO2-ZrO2 system are different from those observed for V2O5/TiO2 catalysts.  Thus, the influence of ZrO2 on the types of surface V species is quite strong.

To examine the thermal properties of precusors of samples their thermal analysis was carried out.  For pure TiO2-ZrO2, the DSC curve show a broad endothermic peak below 383 K due to water elimination, and a sharp and endothermic peak at 976 K due to the TiO2-ZrO2 crystallization.  In the case of 25-V2O5/TiO2-ZrO2 two additional endothermic peaks appear at about 499 and 605 K due to the revolution of NH3 and H2O decomposed from NH4VO3.  Also it is considered that an endothermic peak at 949 K is responsible for the melting of V2O5 and an exothermic peak around 1000 K is due to the formation of new crystalline described in X-ray diffraction pattern.  However, it is of interest to see the influence of vanadium oxide on the phase transition of TiO2-ZrO2 from amorphous to tetragonal phase.  DSC curve showed the exothermic peak due to the phase transition appears at 973 K for pure TiO2-ZrO2, while for V2O5/TiO2-ZrO2 samples it is shifted to lower temperature.  The shift decreases with increasing vanadium oxide content up to 25 wt% of V2O5.  Consequently, the exothermic peaks appear at 874.8 K for 5-V2O5/TiO2-ZrO2, 841.5 K for 10-V2O5/TiO2-ZrO2, and 785.4 K for 25-V2O5/TiO2-ZrO2.

Conclusions

This study has shown that a combination of 51V solid state NMR, XRD, DSC and FTIR can be used to perform the characterization of V2O5/TiO2-ZrO2 prepared by drying a mixed solution of ammonium metavanadate and Ti(OH)4-Zr(OH)4 and calcining in air.  The interaction between vanadium oxide and titania-zirconia influences the physicochemical properties of prpared catalysts with calcination temperature.  On the basis of results of XRD and DSC results the addition of vanadium oxide causes to shift phase transition temperature of ZrO2 from amorphous to tetragonal and tetragonal to monoclinic toward higher temperatures due to the interaction between V2O5 and TiO2-ZrO2.

Reference

1. Y. Nakagawa, T. One, H. Miyata, Y. Kubokawa, J. Chem Soc. Faraday Trans.     1 1983, 79, 2929

2. H. Miyata, M. Kohno, T. Ono, F. Hatayama, J. Chem. Soc. Faraday Trans.

1 1989, 85, 3663