MFI 구조를 갖는 탄탈룸 함유 실리카라이트 분자체의 합성 및 특성연구

고 용 식*,   안 화 승

*신성대학 공업화학과,   인하대학교 화학공학과

Synthesis and characterization of tantalum-containing silicalite molecular sieves

Y. S. Ko, W. H. Ahn / Synthesis of zirconium silicalite-1

with MFI structure

Yong Sig Ko* and  Wha Seung Ahn

*Dept. of Ind. Chem., Sinsung Junior College, Dangjin-gun 343-860, Korea

Dept. of Chem. Eng., College of Eng., Inha University, Inchon 402-751, Korea

Introduction

Isomorphous substitution of silicon and/or aluminum by various hetero-atoms into the framework of different zeolites has received considerable attention in the utilization of zeolites as catalysts.  These elements could impart catalytic properties to the zeolite for many reactions other than those catalyzed by acid sites while offering the advantages of atomic dispersion, which is difficult to achieve with traditional catalyst preparation methods.  In this respect, isomorphous substitution of titanium [1], zirconium [2], or vanadium [3] into the framework of high-silica zeolites has expanded the scope of application of these materials.  Recently, tantalum which belong to the same V-group as vanadium has been reported to have interesting catalytic properties, although only a few reports have been published so far [4,5].  The acid-base properties of tantalum oxide have been studied recently and much interest is shown in Ta2O5 due to its unusual properties as a solid acid catalyst.

The synthesis and characterization of Ti, Zr, or V-silicalite have been extensively studied.  Judging by its ionic radius, Ta5／ can also be incorporated into the MFI structure of silicalite.  Thus, the possibility of incorporating tantalum into silicalite framework has led to the synthesis, for the first time, of a tantalum-containing molecular sieve, TaS-1.

In this work, we report the hydrothermal synthesis and characterization of crystalline tantalum silicalite-1 with the MFI structure using silicon and tantalum alkoxides.  Results from X-ray diffraction, scanning electron microscopy, framework i.r., UV-VIS diffuse reflectance spectroscopy, 29Si MAS NMR, nitrogen adsorption, and elemental analysis were discussed to varify to the incorporation of tantalum in the zeolite lattice framework.

Experimental

1.  Synthesis of tantalum silicalite-1 catalyst

A pyrex beaker was placed in a glove box and nitrogen was flushed over to minimize the adverse effect of moisture in air.  After caculated amount of tetraethylorthosilicate (Aldrich, 98%) was transferred to the pyrex beaker and vigorously stirred, tantalum ethoxide (Aldrich, 99.98%) was carefully introduced into this solution, and the mixture was cooled to about 0℃.  After a few minutes, TPAOH aqueous solution (Aldrich, 20%) also cooled to 0℃ was slowly added dropwise into the mixture.  After addition of all TPAOH the synthesis mixture was kept for 5∼6 hours at 70∼80℃ in order to accelerate hydrolysis and to evaporate the ethyl alcohol which was released.  This clear homogeneous solution was then transferred to a teflon-lined stainless steel autoclave and kept in a convection oven at 175℃ under autogenous pressure.  The solid products were separated by means of suction-filtration, washed several times with hot deionized water and dried in an air oven at 110℃ overnight.  The products were finally calcined at 550℃ for 6 hours.

2.  Characterization

The synthesized samples were analyzed by X-ray diffraction (XRD) for both qualitative and quantitative phase identification.  The unit used was a powder X-ray diffractometer (Philips, PW-1700) with a scintillation counter and a graphite monochromator attachment, utilizing Ni-filtered CuK□ radiation (40kV, 25mA).  Unit cell parameters were obtained by a least-squares fit to the interplanar spacings measured in the 5 ∼ 45°(2□) angular region, using silicon as an internal standard.  The crystal size and morphology of the crystalline samples were examined using a scanning electron microscope (Hitachi, X-650) after coating with a Au-Pd evaporated film.  Framework i.r. spectra of samples were recorded in air at room temperature on a Perkin Elmer 221 spectrometer (in the range of 400 ∼ 4000cm-1) with wafers of zeolites mixed with dry KBr.  The elemental analyses of crystalline samples were performed using ICP spectrometer (Jobin Yuon, JY-38 VHR) and XRF (Rigaku, 3070).  The nitrogen adsorption isotherms and specific surface areas were determined by nitrogen physisorption with the BET method at liquid nitrogen temperature using a Micromeritics ASAP 2000 automatic analyzer.  UV-VIS Diffuse reflectance spectroscopy was performed on a Varian CARY 3E double beam spectrometer and dehydrated MgO was used as a reference in the range of 190 ∼ 500nm.  The solid state NMR spectra were obtained with a Bruker AM 300 spectrometer at a frequency of 59.6 MHz and spinning rate of 3.5 kHz with a pulse width of 3 □sec, a relaxation delay of 5 sec and 100 ∼ 200 acquisitions.  The chemical shift was referenced with respect to the 29Si signal of tetramethylsilane (TMS).

Results and discussion

The isomorphous substitution of Si4／ by larger Ta5／ ions in the TaS-1 framework causes a slight expansion of the unit cell parameters and volume.  The unit cell volume increased linearly with increases in tantalum contents of the reaction mixture up to 3 mol%.  The increase in unit cell volume is due to the introduction of larger Ta5／ ion in the framework lattice.  A similar expansion of the lattice has also been reported for isomorphous substitution of Si4／ by Ti4／, Zr4／ or V5／ in the silicalite framework lattice [1-3].  Again, linear increases in the unit cell volume, V, with the increases in the mole% of Ta suggest the presence of tantalum in the framework.

The overall crystallization rates at constant gel composition and temperature for TS-1, ZrS-1 and TaS-1 are compared.  While the crystallization time was kept to 3 or 4 days to obtain the maximum crystallinity for the TS-1 or ZrS-1, in order to obtain the same degree crystallinity, it was necessary for TaS-1 sample to be kept to 7 days.  The fact that relatively longer crystallization time being necessary to maintain the same degree of crystallinity for TaS-1 may be the consequence of the radius of tantalum atom being larger than that of titanium or zirconium atom, and consequently it is believed to be more difficult for tantalum to enter the silicalite framework than titanium or zirconium.  In addition, while the ZSM-5 which has the same MFI structure can be prepared in 1 day, it takes substantially longer when the titanium, zirconium or tantalum is substituted into the framework.

We have synthesized a series of TaS-1 samples by varying the amount of tantalum in the synthesis gel.  It is seen that the tantalum contents of TaS-1 crystals obtained are linearly correlated with the tantalum contents of the reaction mixtures.  This indicates that varying amounts of tantalum can be incorporated in the crystal by changing the tantalum contents at the stage of gel preparation, and most of the Ta present in the substrate can be incorporated into the zeolite framework within the range of less than about 3 mol%.  The tantalum content in the substrate mixture did not lead to significant modification in the morphology of TaS-1.  All TaS-1 samples were made up of uniform crystals of about 0.3□m and cuboid in shape.  The scanning electron micrograph also confirmed the absence of amorphous matters outside the crystals of TaS-1.

IR spectra of the calcined TaS-1 samples with different tantalum contents are compared in Figure 1.  As the amount of tantalum increases, it was shown that IR absorption bands around 1111cm１1 shifted progressively towards the lower frequency region.  The shift of the spectra to the lower frequency is presumably due to increases in unit cell parameters caused by Ta5／ existing in zeolite lattice framework.  Additionally, IR spectra of TaS-1 showed a characteristic absorption band at 965cm１1, and absorption bands about 950∼970cm１1 are characteristic of metal-oxygen stretching vibrations.  On the other hand, the band was not observed in the IR spectra of calcined ZSM-5, silicalite-1, and Ta2O5 powder.  As can be seen from the IR spectra, the relative intensity of the band at 965cm１1 also increased with increasing tantalum contents.  Similar IR results with TaS-1 have been reported earlier for isomorphous substitution of Si4／ by Ti4／, or V5／ in the silicalite framework [1,6].  In all the cases, an IR band at around 963 or 967cm１1, not present in the Ti, Zr or V free analog, was noticed.  Obviously, the absorption band at 965cm１1 for the TaS-1 should be ascribed to the stretching vibrations of the Ta-O-Si bond.  This suggests that tanatlum ions are incorporated into the framework lattice of the zeolite.

The UV-vis diffuse reflectance spectra of the calcined TaS-1, pure silicalite-1, and Ta2O5 powder are shown in Figure 2.  The UV-vis diffuse reflectance spectrum of TaS-1 exhibited a strong transition band around 200nm similar with ∼210nm of Ti or Zr-silicalite [1,2].  The band around 210nm belongs to a charge transfer band arising from the excitation of an oxygen 2p electron in the valance band to the empty d orbitals of titanium or zirconium ions in the framework of zeolites.  In the Ti or Zr-silicalite, the band around 210nm has been assigned to isolated framework titanium or zirconium in zeolite lattice framework.  The pure silicalite-1 sample showed no corresponding signals.  The main absorption band of Ta2O5 powder was observed at ∼270nm.  The absorption band of TaS-1 sample was significantly different from the band pattern of Ta2O5 powder.  The absence of this band at ∼270nm in TaS-1 samples prepared further confirms the absence of occluded Ta2O5 in the framework.

29Si MAS NMR spectrum of the TaS-1 was very different to those of pure silicalite-1, Ti-silicalite, and Zr-silicalite.  The spectrum consists of a main signal at about １113ppm and separate small peaks.  Following the discussion in literature the main peak at １113ppm can be assigned to Si(4Si) coordination.  The signal at about １103ppm is indicative of Si(OSi)3O１ framework defects.  This seems reasonable in particular in the case of Ta-silicalite 1 where at least a partial incorporation of the large Ta5／ ions should lead to structural distortions and defects of the silicalite structure.  The broadening of lines in the spectrum of V-silicalite can be an indirect indication of the statistical distribution of the vanadium in the lattice.  29Si MAS NMR spectra of Me (Ti, Zr, Ge)-silicalite samples usually show broadening of the signals with increasing Me content which is connected with the increasing degree of distortion of the silicalite lattice.  Considering above literatures, the 29Si MAS NMR results indicated that the added tantalum ions have been incorporation into the framework lattice of the zeolite.

References

[1]  Ko, Y.S. and Ahn, W.S., Korean J. Chem. Eng., 15(2), 182 (1998).

[2]  Ko, Y.S. and Ahn, W.S., Korean J. Chem. Eng., 15(4), 423 (1998).

[3]  Prasad Rao, P.R.H., Belhekar, A.A., Hegde, S.G., Ramaswamy, A.V. and

Ratnasamy, P., J. Catal., 141, 595 (1993).

[4]  Tanabe, K., Misono, M., Ono, Y. and Hattori, H., Stud. Surf. Sci. Catal., 51,

Delmon, B. and Yates, J.T., eds., Elsevier, New York, 64, 1989.

[5]  Ushikubo, T. and Wada, K., Appl. Catal., 67, 25 (1990).

[6]  Reddy, K.R., Ramaswamy, A.V. and Ratnasamy, P., J. Catal., 143, 275 (1993).

Fig.2. UV-vis diffuse reflectance spectra of

(a) silicalite-1, (b) TaS-1, (c) Ta2O5

Fig.1. IR spectra of TaS-1 synthesized with

tantalum contents:

(a) 1mol%, (b) 2mol%, (c) 3mol%