Synthesis and characterisation of microporous titanoniobosilicate ETNbS-10
Joa˜o Rocha,*a Paula Branda˜o,a Ju´lio D. Pedrosa de Jesus,a Andreas Philippoub and Michael W. Andersonb
a Department of Chemistry, University of Aveiro, 3810 Aveiro, Portugal. E-mail: rocha@dq.ua.pt
b Department of Chemistry, UMIST, PO Box 88, Manchester, UK M60 1QD
Received (in Cambridge, UK) 24th December 1998, Accepted 2nd February 1999
The synthesis, structural characterisation and catalytic
activity in tert-butanol dehydration of microporous titano-
niobosilicate ETNbS-10 are reported.
Due to this broadening, the ETS-10 line splitting at d ca. 296.5
is no longer resolved. This seems to indicate the framework
insertion of Nb because studies on Al-substituted ETS-10 have
shown that the broadening of the 29Si MAS NMR resonances is
due to lattice distortion upon Al incorporation.6 However, in
ETNbS-10 Nb is likely to replace Ti and not Si. We note in
passing that ETS-10 and an ETS-10 sample impregnated with
niobium give very similar 29Si MAS NMR spectra.
ETS-10 is a novel microporous framework titanosilicate
consisting of ‘TiO2’ rods, which run in two orthogonal
directions, surrounded by tetrahedral silicate units.1–3 The pore
structure consists of 12-rings, 7-rings, 5-rings and 3-rings and
has a three-dimensional wide-pore channel system whose
minimum diameter is defined by 12-ring apertures. In an
attempt to improve the acid characteristics of ETS-10, silicon
has been isomorphously substituted by boron, aluminium and
gallium.4–6 A vanadosilicate with the structure of ETS-10 has
also been reported.7 We now wish to report the synthesis,
structural characterisation and catalytic activity in the dehydra-
tion of tert-butanol, of niobium-substituted ETS-10.
ETNbS-10 materials with Nb/Ti molar ratios (ICP) of 0.12,
0.38 and 0.47 were prepared using a modification of the ETS-10
synthesis. An alkaline solution was made by mixing 10.00 g
sodium silicate (Na2O 8% m/m, SiO2 27% m/m, Merck), 15.03
g H2O, 0.80 g NaOH (Merck), 0.96 KF (Aldrich), 0.38 g KCl
(Panreac), 1.02 g NaCl (Aldrich) 4.63 g TiCl3 (15% m/m
solution of TiCl3 in 10% m/m HCl, Merck) and 1.50 g
Nb(HC2O4)5 (Niobium Products). 0.10 g seed of ETS-10 was
added to the resulting gel. This gel, with a composition 5.6
Na2O:2.4 K2O:10.0 SiO2 :TiO2 :0.30 Nb2O5 :266 H2O, was
autoclaved under autogeneous pressure for 3 days at 230 °C.
The crystalline product was filtered, washed with distilled water
and dried at ambient temperature, the final product being an off-
white microcrystalline (2–3.5 mm) powder. The samples were
characterised by powder X-ray diffraction (XRD), high-
resolution and scanning electron microscopies (HREM and
SEM, respectively), infrared (TFIR) and Raman spectroscopies,
29Si and 93Nb solid state NMR.
The central-transition 93Nb MAS NMR spectrum [Fig. 1(b)]
of ETNbS-10 contains a broad (full-width-at-half-maximum,
FWHM, ca. 240 ppm) asymmetric resonance at d ca. 100
relative to solid Nb2O5. This is consistent with niobium being
present in distorted octaedral coordination.8
Raman spectra (Fig. 2) provide perhaps the best evidence for
the isomorphous substitution of Ti by Nb in the ETS-10
framework. ETS-10 gives a main strong and sharp band at ca.
735 cm21, assigned to the TiO6 octahedra. As the samples Nb
content increases this peak shifts slightly and broadens and,
simultaneously, a band grows at ca. 664 cm21. The latter is
typical of NbO6 octahedra in microporous niobosilicates:
titanoniobosilicate synthetic analogues of the mineral nenadke-
vichite give a similar band at 668 cm21 8
, while a recently
reported niobosilicate (AM-11) gives a band at 687 cm21.9 The
FTIR spectrum of ETS-10 (not shown) displays bands at 446,
550 and 746 cm21, associated with the TiO6 octahedra. As the
Nb content of the samples increases the intensity of these bands
decreases, while a new band at 918 cm21 is seen. This is a
The ETNbS-10 samples with Nb/Ti 0.12 and 0.38 are
essentially pure. SEM and powder XRD reveal that the sample
with Nb/Ti 0.47 also contains some quartz ( < 10%) and, in
addition, a small amount of an unknown niobosilicate. The
HREM images of this sample (not shown) and ETS-102,3 are
very similar. The powder XRD patterns of ETS-10 and ETNbS-
10 samples (not shown) are virtually identical and show no
significant shift in any reflection. This is to be expected because
Ti(iv) and Nb(v) have very similar ionic radii.
The total mass losses between 30 and 700 °C of ETNbS-10
(ca. 11%, Nb/Ti 0.12) and ETS-10 (ca. 12.5%) are similar.
Nitrogen adsorption isotherms of ETNbS-10 and ETS-10
materials are of type I with maximum uptakes of ca. 0.11 and
0,12 g/g, respectively. However, due to the presence of quartz
and niobosilicate dense impurities the maximum uptake of
ETNbS-10 with Ti/Nb 0.47 is much smaller (ca. 0.06 g/g).
Fig. 1(a) shows the 29Si magic-angle spinning (MAS) NMR
spectra of ETS-10 and ETNbS-10 (Ti/Nb 0.47). In ETS-10 there
are two types of silicon chemical environments, Si(3Si, 1Ti) and
Si(4Si, 0Ti), which give the two groups of resonances at d 294
to 297 and d ca. 2103.7, respectively.3 the spectrum reveals a
further crystallograpic splitting of the Si(3Si, 1Ti) site. ETNbS-
10 gives a 29Si MAS NMR spectrum similar to that of ETS-10.
However, all resonances broaden considerably and, in particular
the ETS-10 peak at d 2103.7, shifts slightly to low frequency.
Fig. 1 (a) 29Si MAS NMR spectra of ETS-10 and ETNbS-10 (Nb/Ti 0.47).
(b) 93Nb MAS NMR spectra of Nb2O5 and ETNbS-10 (Nb/Ti 0.47). The
29Si and 93Nb spectra were recorded at 79.5 and 97.84 MHz on a Bruker
MSL 400P spectrometer using spinning rates of 5 and 32 kHz, re-
spectively.
Chem. Commun., 1999, 471–472
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