introduction · various heteroatoms into the framework of different zeo-lites has received...

Introduction1)

Isomorphous substitution of silicon and/or aluminum by

various heteroatoms into the framework of different zeo-

lites has received considerable attention in the utilization

of zeolites as catalysts. These elements can impart cata-

lytic properties to the zeolite for many reactions other

than those catalyzed by acid sites while offering the ad-

vantages of atomic dispersion of the elements, which is

difficult to achieve with traditional catalyst preparation

methods. In this respect, isomorphous substitution of tita-

nium, zirconium, or vanadium into the framework of

high-silica zeolites has expanded the scope of application

of these materials. These isomorphously substituted zeo-

lites are reported to be very promising in the liquid-phase

catalytic oxidation of hydrocarbons. Taramasso and cow-

†To whom all correspondence should be addressed.

(e-mail: [email protected])

orkers [1] first reported the hydrothermal synthesis of a

titanium-containing zeolite, denoted as titanium silica-

lite-1 (TS-1). Titanium silicalite-1 was found to be active

in the oxidation of a variety of organic substrates in the

presence of aqueous hydrogen peroxide as oxidizing

agent. Much the same as titanium-substituted molecular

sieves, zirconium or vanadium silicalite-1 (ZS-1 or VS-

1) can be synthesized by isomorphous substitution of

Zr4+or V

5+for Si

4+in the MFI structure framework, and

has very interesting properties towards catalytic oxida-

tion [2-7]. The oxidative property of vanadium could be

combined with the shape-selective characteristics im-

parted by the micropore structure in which the vanadium

is located [8].

Niobium - which belongs to the same group in the

Periodic Table as vanadium (Group V) - has been re-

ported to have the capability of the photocatalytic and se-

lective oxidations [9-11]. Although electronegativity and

ionic radius between niobium and its neighbors (V, Mo,

Yong Sig Ko†, Hyun Tae Jang , andWha Seung Ahn

Department of Advanced Material Chemistry, Shinsung College, Dangjin-gun 343-860, Korea

*Department of Chemical Engineering, Hanseo University, Seosan 356-706, Korea

**Department of Chemical Engineering, College of Engineering, Inha University, Inchon 402-751, Korea

Received January 29, 2007; Accepted May 3, 2007

Abstract:Niobium silicalite-1 (NbS-1) molecular sieves with MFI structure were prepared using the hydro-

thermal synthesis method. X-ray diffraction, scanning and transmission electron microscopies, Fourier transform

infrared spectroscopy, ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy,29Si magic-angle spinning

nuclear magnetic resonance spectroscopy, ammonia temperature-programmed desorption (NH3-TPD), physical

adsorption of nitrogen and elemental analysis were then performed to evaluate their physico-chemical properties

and to provide evidences for the incorporation of Nb5+into the zeolite framework. The unit cell volume of niobi-

um silicalite-1 increased linearly with increasing niobium content, suggesting isomorphous substitution of Si4+

by Nb5+in the lattice framework. The framework infrared spectrum showed a characteristic absorption band at

963 cm-1, probably due to Si-O-Nb linkages. In the UV-vis spectrum, the absorption assigned to the presence of

Nb5+

in the zeolite framework was observed at around 200 nm. In addition, NH3-TPD studies revealed weak

acidity in niobium silicalite-1 due possibly to [Nb (OH)-Si] sites in the framework.․․․

Keywords: niobium, niobium silicalite-1, hydrothermal synthesis, NH3-TPD, isomorphous substitution, charac-

terization, FT-IR, UV-vis DRS

Hydrothermal Synthesis and Characterization of Niobium-containing Silicalite-1 Molecular Sieves with MFI Structure 765

Ta, and Zr) in the Periodic Table are different, it is re-

ported that the promoter effect, support effect and acidic

nature of niobium compounds are quite different from

those of compounds of the surrounding elements [12].

Niobium compounds and niobium materials have re-

cently been shown to enhance catalytic properties when

used as a component of catalysts or when small amounts

are added to known catalysts. Depending on the catalyst

composition and structure, they are used in the processes

involving acidic active centers as well as redox character.

These niobium-containing catalysts have been used for

dehydration and dehydrogenation of alcohols [13-15],

oxidative dehydrogenation of alkanes [16], alkylation of

benzene [17], NO reduction by NH3 [18], dimerization

and oligomerization of olefins [19], oxidation of meth-

anol [20,21] and polycondensation reactions [22]. The

presence of niobium in molecular sieves has the potential

of generating catalysts with shape-selective properties

and acidic or redox characteristics. Only a few reports

are available on porous niobium silicates; they include

work on the incorporation of niobium in micro/meso-

porous materials, ETS-10 [23] denoted as ETNbS-10, to

MFI [24] designated as NbS-1, and also to MCM-41

[25].

Though synthesis methods of niobium silicate molec-

ular sieves have been reported [24,26], further inves-

tigation concerning the hydrothermal synthesis of niobi-

um-containing silicalite molecular sieves with more de-

tailed characterization would contribute towards a better

understanding of the system.

In this work, we report the hydrothermal synthesis and

characterization of crystalline niobium silicalite molec-

ular sieves with the MFI structure prepared using silicon

and niobium alkoxides. Results from X-ray diffraction

(XRD), scanning electron microscopy (SEM), transmi-

ssion electron microscopy (TEM), framework infrared

(IR) spectroscopy, ultraviolet-visible (UV-vis) diffuse re-

flectance spectroscopy (DRS),29Si magic-angle spinning

(MAS) nuclear magnetic resonance spectroscopy

(NMR), temperature-programmed desorption of ammo-

nia (NH3-TPD), nitrogen adsorption and elemental analy-

sis are discussed to verify the incorporation of niobium

in the zeolite lattice framework.

Experimental

Synthesis of niobium silicalite-1 catalyst

NbS-1 samples were prepared from substrates having

the following composition ratio: SiO2/Nb2O5 = 32.7∼

99, TPA+/SiO2 = 0.46, H2O/SiO2 = 35. In a typical prepa-

ration, a Pyrex beaker was placed in a glove box and ni-

trogen was flushed over to minimize the adverse effect of

moisture in air. After 27.01 g of tetraethylorthosilicate

Figure 1. Schematic diagram for the hydrothermal synthesis of

niobium silicalite-1.

(Aldrich, 99.999 %) was transferred to the Pyrex beaker

and vigorously stirred, 0.43 g of niobium(V) ethoxide

(Aldrich, 99.95 %) was carefully introduced into this sol-

ution, and the mixture was cooled to about 273 K. After

a few minutes, 60.65 g of tetrapropylammonium hydrox-

ide (TPAOH) aqueous solution (Aldrich, 20 %), also

cooled to 273 K, was slowly added dropwise into the

mixture. Stirring and cooling were maintained during this

process. After addition of all TPAOH the synthesis mix-

ture was kept for 5 6 h at 343 353 K in order to accel∼ ∼ -

erate hydrolysis and to evaporate the ethyl alcohol

produced. Deionized water was then added to increase

the volume of the mixture to its original value. This clear

homogeneous solution was then transferred to a Teflon-

lined stainless steel autoclave and kept in a convection

oven at 448 K under autogenous pressure. In order to in-

vestigate the crystallization process, autoclaves were tak-

en out from the oven at different time intervals and were

quenched immediately in cold water for sample identifi-

cation. The solid products were separated by means of

suction-filtration or centrifugation, washed several times

with hot deionized water and dried in an air oven at 383

K overnight. The products were finally calcined at 823 K

for 6 h. The synthesis procedure of niobium silicalite-1

catalyst can be represented schematically as in Figure 1.

A reference sample of silicalite-1 (MFI) was prepared

following the same procedure without adding niobium

ethoxide to the substrate mixture. In addition, a sample

Yong Sig Ko, Hyun Tae Jang, and Wha Seung Ahn766

Figure 2. XRD patterns of niobium silicalite-1 and silicalite-

1 (a) silicalite-1, (b) ZSM-5, (c) NbS-1 (3 mol% niobium),：

and (d) Nb2O5 powder.

of about 3 mol% niobium-impregnated silicalite-1 [Nb/

silicalite-1 (IMP)] was also prepared for comparison pur-

poses by the incipient wetness method.

Characterization

The samples synthesized were analyzed by X-ray dif-

fraction (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, uti-

lizing nickel-filtered CuK radiation (40 kV, 25 mA).α

Unit cell parameters were obtained by a least-squares fit

to the interplanar spacings measured in the 5 45° (2 )∼ θ

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. For TEM analysis, samples were dispersed ultra-

sonically in ethanol, and a drop of the suspension was

deposited on a holey carbon copper grid. Micrographs

were recorded with a 1024 × 1024 Gatan CCD camera on

a Philips CM20 microscope operated at 200 kV. Frame-

work infrared (IR) spectra of samples were recorded in

air at room temperature on a Perkin Elmer 221 spec-

trometer (in the range of 400 4000 cm∼-1) with wafers of

zeolites mixed with dry KBr. The elemental analyses of

crystalline samples were performed with an inductively

coupled plasma (ICP) spectrometer (Jobin Yuon, JY-38

VHR) and X-ray fluorescence (XRF; Rigaku, 3070).

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

190 500 nm. The solid-state NMR spectra were ob∼ -

tained with a Bruker AM 300 spectrometer at a fre-

quency of 59.6 MHz and spinning rate of 3.5 kHz with a

pulse width of 3 µs, a relaxation delay of 5 s and 100∼

200 acquisitions. The chemical shift was referenced with

respect to the29Si signal of tetramethylsilane (TMS). The

nitrogen-adsorption isotherms and specific surface areas

were determined by nitrogen physisorption with the

Brunauer-Emmett-Teller (BET) method at liquid-nitro-

gen temperature using a Micromeritics ASAP 2010 auto-

matic analyzer. Ammonia temperature-programmed de-

sorption (NH3-TPD) tests were carried out in a quartz mi-

croreactor with continuous analysis of the released NH3concentration via a TCD detector using a Micromeritics

TPD/TPR 2900 analyzer. Before the tests, the sample

was pretreated in situ with a flow of argon at 773 K in or-

der to remove adsorbed species, and the adsorption was

conducted after the sample was cooled to 373 K. The

physisorbed NH3 was flushed by a dry argon flow at the

same temperature for 3 h. The temperature was then in-

creased at the rate of 10 K min-1.

Results and Discussion

The X-ray diffraction patterns of calcined NbS-1, ZSM-

5, silicalite-1, and N2O5 powder are shown in Figure 2.

The X-ray diffraction pattern of the calcined NbS-1 mo-

lecular sieve was similar to that of ZSM-5 zeolite with

MFI structure. Almost identical diffractograms of NbS-1

were obtained irrespective of the niobium contents of the

sample up to 3 mol% and no detectable niobium oxide

peaks were observed. The symmetry of the calcined

NbS-1 was orthorhombic, comprising single reflections

around 2 at 24.5θoand 29.2

o. As expected, calcined sili-

calite-1 had monoclinic symmetry which presents double

reflections at the same 2θ. The persistence of the ortho-

rhombic symmetry in the calcined state of the niobium

silicalites can be considered as a result of niobium ions

present in the framework positions in NbS-1.

The isomorphous substitution of Si4+

by larger Nb5+

ions in the NbS-1 framework causes a slight expansion

of the unit cell parameters and volume. Figure 3 shows

the variation of unit cell volume calculated for calcined

NbS-1 samples with different niobium contents. The unit

cell volume increases linearly with increasing niobium

content of the reaction mixture up to 3 mol%. The in-

crease in unit cell volume is due to the introduction of

larger Nb5+

ion in the framework lattice. A similar ex-

pansion of the lattice has also been reported for iso-

morphous substitution of Si4+

by Ti4+, Zr

4+or V

5+in the

silicalite framework lattice [1,2,5]. Again, linear in-

creases in the unit cell volume, V, with increases in the

amount (mol%) of niobium suggest the presence of


Figure 3. Unit cell volume of NbS-1 as a function of niobium

content.

Figure 4. Comparison between the crystallization rates of TS-1,

ZrS-1 and NbS-1.

niobium in the framework. Earlier studies [27,28] have

shown that the expansion of the unit cell volume is re-

lated to the concentration of the framework vanadium in

the V5+containing zeolite samples.

The overall crystallization rates at constant gel compo-

sition and temperature for TS-1, Zr-containing zeolite

ZrS-1 [29] and NbS-1 are compared in Figure 4. While

the crystallization time was kept to 3 or 4 days to obtain

the maximum crystallinity for TS-1 or ZrS-1, it was nec-

essary for NbS-1 sample to be kept to 10 days in order to

obtain the same degree crystallinity. The fact that a rela-

tively longer crystallization time was necessary to main-

tain the same degree of crystallinity for NbS-1 may be a

consequence of the radius of niobium atoms being larger

than that of titanium or zirconium atoms, and conse-

quently it is believed to be more difficult for niobium to

Figure 5. Relationship between the niobium content of the

NbS-1 samples and the gel mixtures.

enter the silicalite framework than titanium or zirconium.

In addition, while ZSM-5 (which has the same MFI

structure) can be prepared in 1 day, it takes substantially

longer when titanium, zirconium or niobium is sub-

stituted into the framework. Such a trend is generally ob-

served in the synthesis of metal-substituted silicate mo-

lecular sieves [29-31], but it can be also considered as a

consequence of the mineralizing agent NaOH/KOH be-

ing absent in the reaction mixture in such cases.

We have synthesized a series of NbS-1 samples by

varying the amount of niobium in the synthesis gel.

Figure 5 shows the bulk niobium contents of NbS-1 crys-

tals plotted against the niobium content of their reaction

mixtures. It is seen that the niobium content of the NbS-1

crystals obtained is linearly correlated with the niobium

content of the reaction mixtures. This indicates that vary-

ing amounts of niobium can be incorporated in the crys-

tal by changing the niobium content at the stage of gel

preparation, and most of the niobium present in the sub-

strate can be incorporated into the zeolite framework

within the range of less than about 3 mol%.

A scanning electron micrograph and transmission elec-

tron micrograph of NbS-1 (3 mol% niobium) sample are

presented in Figure 6. The niobium content in the sub-

strate mixture did not lead to a significant modification

in the morphology of NbS-1. All NbS-1 samples were

made up of uniform crystals of size about 0.13 µm and

hexagonal shape. The transmission electron micrograph

of the NbS-1 sample shows the absence of amorphous

matters outside the crystals of NbS-1.

IR spectra of the calcined silicalite-1 and NbS-1 are

compared in Figure 7. As is known, IR spectra in the

mid-infrared region can provide evidence of the iso-

morphous substitution of large heteroatoms into the

framework of zeolites [32,33]. The band around 1100


(a) (b)

Figure 6. SEM photograph (a) and TEM image (bar = 100 nm) (b) of the calcined niobium silicalite-1 (3 mol% niobium).

Figure 7. IR spectra of the calcined (a) silicalite-1 and (b)

NbS-1 (3 mol%).

cm-1corresponds to asymmetric vibrations of internal tet-

rahedra, and the symmetric stretching of T-O is found

around 820 780 cm∼-1[34]. The IR spectrum of NbS-1

showed a characteristic absorption band at 963 cm-1,

which may be ascribed to Si-O-Nb stretching vibrations.

This band was not observed in the IR spectra of calcined

ZSM-5, pure silicalite-1 or in Nb2O5 powder. An absorp-

tion band near 950 970 cm∼-1is usually known as char-

acteristic of metal-oxygen stretching vibrations [27,

35,36] or it has been interpreted to be a consequence of

the Si-O-vibrations [32] perturbed by metal ions nearby

in the zeolite framework. Similar IR results have been re-

ported earlier for isomorphous substitution of Si4+

by

Ti4+, Zr

4+, and V

5+in the silicalite framework [1,2,8] ; in

all these cases, an IR band around 963 or 967 cm-1, not

Figure 8. UV-vis diffuse reflectance spectra of (a) silicalite-1,

(b) NbS-1, (c) Nb2O5 powder and (d) Nb/silicalite-1 (IMP).

present in the titanium-, zirconium- or vanadium-free an-

alog was observed.

The UV-vis diffuse reflectance spectra of the calcined

NbS-1, Nb/silicalite-1 (IMP), pure silicalite-1 and Nb2O5powder are shown in Figure 8. Quite similar spectra were

obtained among NbS-1 samples with niobium content up

to 3 mol%. The absorption band of the NbS-1 sample

was significantly different from the band pattern of

Nb/silicalite-1 (IMP) or Nb2O5 powder. The UV-vis dif-

fuse reflectance spectrum of NbS-1 exhibited a strong

transition band around 200 nm, similar to the band at ∼

210 nm of titanium- or zirconium-containing silicalite

[4,32]. The band around 210 nm is a chargetransfer band

arising from excitation of an oxygen 2p electron in the

valance band to the empty d orbitals of titanium [6,36] or

zirconium [4] ions in the framework of zeolites. In the ti-

tanium or zirconium silicalite, the band around 210 nm

has been assigned to isolated titanium or zirconium in

zeolite lattice framework [4,32]. The pure silicalite-1

sample showed no corresponding signals. Thus, we sug-


Figure 9.29Si MAS NMR spectra of the calcined (a) silicalite-1

and (b) NbS-1.

gest that the absorption band around 200 nm for NbS-1 is

due to Si-O-Nb units in the framework. The main absorp-

tion band of Nb2O5 powder was observed at 310 nm,∼

and the absorption band of Nb/silicalite-1 (IMP) was

very similar to that of the Nb2O5 powder. The absence of

this band at 310 nm in NbS-1 samples prepared further∼

confirms the absence of occluded Nb2O5 in the frame-

work.

Figure 9 shows the solid state29Si MAS NMR spectra

of the calcined NbS-1 and pure silicalite-1 samples. The29Si MAS NMR spectrum of the NbS-1 was very similar

to that of TS-1, ZrS-1 or VS-1 [37-39]. The NMR spec-

trum of the NbS-1 displays a resonance at about -103

ppm, a main signal at -113 ppm and a shoulder at about

-116 ppm. Following the discussion in literature [40,41]

the main peak at -113 ppm can be assigned to the central

Si atom (in bold type) in Si(OSi)4 (Q4) sites based on the

chemical shift. The29Si MAS NMR spectrum of NbS-1

showed a characteristic shoulder around -116 ppm which

did not appear in the case of pure silicalite-1. In an ear-

lier study on TS-1, this signal was assigned to (TiO)Si

(OSi)3 units and cited as evidence for framework sub-

stitution of titanium [37]. Thus, we suggest that the sh-

oulder at about -116 ppm of the NbS-1 is presumably at-

tributed to the distorted silicon environment in tetrahe-

dral due to Si-O-Nb bond. Axon and Klinowski [41] re-

ported that the signal at about -103 ppm is indicative of

Si(OSi)3O-(Q

3unit) framework defects. This seems rea-

sonable in particular in the case of niobium silicalite-1

where at least a partial incorporation of the large Nb5+

ions should lead to structural distortions and defects of

Figure 10. Nitrogen-adsorption isotherms of the calcined silica-

lite-1 and NbS-1.

Figure 11. Specific surface area (BET) of NbS-1 with different

niobium contents.

the silicalite structure.

Figure 10 shows the nitrogen adsorption-desorption iso-

therms of NbS-1 and pure silicalite-1 samples calcined at

823 K in N2. Both isotherms were of type I frequently

found in zeolitic sorbents. As can be seen, the nitrogen-

adsorption capacity of NbS-1 was very similar to that of

the pure silicalite-1 and indicated the absence of oc-

cluded matter like Nb2O5 within the pore system. The

shapes of the two isotherms were also similar except for

the presence of a large high-pressure hysteresis loop in

silicalite-1, which results in a lower micropore volume.

Figure 11 shows the specific surface area (BET) of

NbS-1 with different amounts (mol%) of niobium in the

reaction mixture. The specific surface area of NbS-1 was

about 504.5 m2g-1and remained almost constant up to 3

mol%. Therefore, it could be speculated that no occluded

niobium species are present in the samples prepared.

From the results of N2-adsorption measurements on

NbS-1 samples, it could be seen that the majority of the

niobium up to 3 mol% in the reaction mixture was sub-

stituted isomorphously into the zeolite framework.

In order to study the potential development of acidity in

the niobium silicalite-1 sample, NH3-TPD tests were car-


Figure 12. NH3-TPD profiles for ammonia desorption from (a)

silicalite-1, (b) Nb/silicalite-1 (IMP), and (c) NbS-1.

ried out on the pure silicalite-1, Nb/silicalite-1 (IMP) and

NbS-1. The results obtained are shown in Figure 12. As

expected, acid sites were almost absent in pure silica-

lite-1. A broad NH3 desorption peak positioned between

400 and 600 K was observed in the NbS-1 sample. This

corresponds to the weak acid sites, which may be attrib-

uted to silanol groups located near to niobium sites [Nb․․․

(OH)-Si] or due to defect site created within the zeolite

structure as Nb-OH. On the other hand, the Nb/silica-

lite-1 (IMP) produced a far smaller desorption peak cen-

tered near 470 K, despite almost the same niobium∼

content. This desorption peak is probably associated with

Nb-OH of the extraframework niobium oxides in the

sample. The NH3-TPD results further prove that Nb5+

species in the niobium silicalite-1 sample were incorpo-

rated into the silicalite framework.

Conclusion

Niobium silicalite-1 (NbS-1) molecular sieves with the

MFI structure have been synthesized hydrothermally,

isomorphously substituting Nb5+

for the framework Si4+

of silicalite. Uniform-sized (0.13 µm) and hexagonal-

shaped crystals were obtained, and the unit cell volume

of NbS-1 lattice was found to increase linearly with in-

creasing niobium content of the substrate. Framework IR

spectrum showed a characteristic absorption band at

around 963 cm-1, probably due to Si-O-Nb linkages. In

the UV-vis spectrum, the absorption around 200 nm,

which was assigned to the presence of Nb5+in the zeolite

framework, was observed for the NbS-1. The29Si MAS

NMR spectrum of NbS-1 exhibited a well-defined shoul-

der around -116 ppm due to the distorted Si environment.

Finally, NH3-TPD studies revealed a development of

mild acid sites in NbS-1. All the diverse characterization

methods employed in this study confirmed the structural

incorporation of niobium in NbS-1.

Acknowledgment

This research was supported by a grant (code DD2-101)

from Carbon Dioxide Reduction & Sequestration Re-

search Center, one of the 21st Century Frontier Programs

funded by the Ministry of Science and Technology of

Korean government.

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introduction · various heteroatoms into the framework of different zeo-lites has received...

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