Synthesis and characterisation of a novel microporous niobium silicate catalyst
a
a
b
b
Jo a˜ o Rocha,* Paula Brand a˜ o, Andreas Phillippou and Michael W. Anderson
a
Department of Chemistry, University of Aveiro, 3810 Aveiro, Portugal. E-mail: ROCHA@DQ.UA.PT
Department of Chemistry, UMIST, PO Box 88, Manchester, UK M60 1QD
b
Received (in Cambridge, UK) 26th October 1998, Accepted 6th November 1998
The synthesis and characterisation of a novel microporous
niobium silicate (AM-11), an excellent catalysts for the
conversion of alcohols, are reported.
the parent and calcined hydrated materials were found to be
identical.
The 29Si MAS NMR spectrum of AM-11 [Fig. 2(a)] displays
a resonance at d 295.6, a group of (at least) three overlapping
peaks at d 2105.5, ca. 2106.7 and 2108.1 and a sharp signal
at d 2111.1. The attribution of these resonances is difficult
because, to the best of our knowledge, no systematic study is
Recently, the synthesis of microporous framework titanium
silicates, displaying zeolite-type properties and containing
Ti(iv) usually in octahedral coordination, has attracted much
interest.1 As a natural extension of this work, we have
embarked on a systematic study aimed at preparing novel
–4
29
available on the relationship between the Si NMR chemical
shift and the number of niobium polyhedra coordinating a given
silicon tetrahedron. Our previous study on synthetic analogues
5
6
microporous zirconium and niobium silicates. Here we report
the synthesis and structural characterisation of a novel micro-
porous niobium silicate denoted AM-11 (Aveiro-Manchester
microporous solid no. 11).
6
of the mineral nenadkevichite, containing framework niobium,
suggests that this relationship may be similar to that found for
titanium silicates where a systematic downfield chemical shift is
observed when increasing numbers of titanium polyhedra
AM-11 was prepared in Teflon-lined autoclaves under static
hydrothermal conditions. An alkaline solution was made by
mixing 1.27 g tetraethylorthosilicate (Aldrich), 2.40 g ethanol,
7
coordinate a given silicon tetrahedron. We assign the peak at d
2111.1 to Si(4 Si, 0 Nb) environments. The resonances at d
6
.40 g H
2
O, 1.68 g NaOH (Merck) and 0.53 g NaF (Aldrich). A
O, 3.00 g oxalic
2105.5, ca. 2106.7 and 2108.1 are tentatively assigned to Si(4
3
second solution was made by mixing 20.0 g H
2
Si, 0 Nb) or Si(3 Si, 1 Nb) environments, while that at d 295.6
acid (Panreac), 1.00 g niobium oxalate (Niobium Products) and
stirred overnight. These two solutions were combined, seeded
is attributed to Si(3 Si, 1 Nb) or Si(2 Si, 2 Nb) sites.
The central-transition 93Nb MAS NMR spectrum [Fig. 2(b)]
of AM-11 contains a broad (full-width at half maximum,
FWHM, ca. 127 ppm) resonance centred at d 2100 relatively
1
with 0.1 g ETS-4 and stirred thoroughly. The pH (after a 1:100
water dilution) was adjusted to 10.2 by adding an ammonia
solution (25%, Merck). The gel, with a composition 1.0
2 5
to solid Nb O (the other peaks are spinning sidebands).
93
2
Na O:1.0 SiO
2
:0.15 Nb
O
2 5
2
:240 H O, was autoclaved for 15
Considering the few Nb NMR spectra reported for niobium
days at 200 °C. The crystalline product was filtered off, washed
with distilled water and dried at ambient temperature, the final
product being an off-white microcrystalline powder.
AM-11 samples were characterised by powder X-ray diffrac-
9
3
29
tion (XRD), scanning electron microscopy (SEM), Nb, Si
and 23Na magic-angle spinning nuclear magnetic resonance
(
MAS NMR), Raman spectroscopy, thermogravimetry (TG),
adsorption isotherms and catalytic tests.
SEM (not shown) reveals that AM-11 crystals are needles ca.
1
0 mm in length. Energy dispersive absorption of X-rays yields
Si/Nb and Na/Nb molar ratios of ca. 4.6 and 0.3, respectively.
The total AM-11 mass loss between 30 and 700 °C is ca.
1
6.5%. Two stages of dehydration are observed (not shown):
between 30 and 270 °C the solid loses ca. 10.5% while between
70 and 700 °C the mass loss is ca. 6%. The adsorption
2
behaviour of AM-11 was investigated in order to assess the
porosity of the material. Nitrogen and methanol adsorption
isotherms are both type I with maximum uptakes of ca. 0.15 and
Fig. 1 Powder XRD pattern of AM-11.
Table 1 Powder XRD data of AM-11
2
1
0
.11 g g , respectively. Although the material is microporous,
the propane uptake was found to be negligible suggesting a very
d/Å
I/I
0
d/Å
I/I
0
small pore size. However, ethanol, n-propanol and 2-methylpro-
2
1
0
11.861
64
4
14
80
3
3.421
3.279
3.165
3.080
2.959
2.873
2.810
2.715
2.584
2.523
2.475
2.323
2.237
2.198
2.125
63
7
pan-1-ol have uptakes of ca. 0.6–0.7 g g at P/P = 0.5. This
conflicting behaviour may originate in the polarity of the
sorbate.
8.964
7.114
6.841
6.489
6.337
5.923
85
15
100
23
7
5
36
8
18
12
20
6
The powder XRD pattern of AM-11 is shown in Fig. 1, while
the d-spacings and intensities of the main reflections are
collected in Table 1. Peak intensities were altered by changing
the method of sample preparation indicating the occurrence of
preferred orientation effects. In situ powder XRD patterns
recorded under vacuum at temperatures up to 650 °C (not
shown) are similar to the pattern given by the parent hydrated
material. The water in AM-11 is zeolitic and its removal is not
permanent. This was confirmed by first heating a sample at 650
3
34
15
61
6
17
6
8
12
43
4.941
4.474
4.243
3
3
3
3
3
.940
.860
.827
.750
.571
°
C and then keeping it, for several hours, in contact with air at
20
room temperature. The powder XRD patterns and TG curves of
Chem. Commun., 1998, 2687–2688
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