20
U. Filek et al. / Catalysis Communications 30 (2013) 19–22
the salts was carried out at 50 °C. The crystals formed were quickly
rinsed with the cold water, dried in air at ambient temperature and
kept in a desiccator over the magnesium nitrate solution.
were found, in particular Ga(NO3)3·8H2O [ICSD 00-012-0398] and
Ga2O3 [ICSD 04-004-5292]. GaPW is monoclinic and belongs to the
space group P21/c. The unit cell constants are listed in Table 1.
The indium InPW salt shows the strongest reflections at 2θ=7.9°,
9.0°, 18.1°, 20.7°, 26.0°, 26.8° and 33.4° (Fig. 1c). No other phases
were found in this sample as well, in particular a hydrated indium ni-
trate, NO(In(NO3)4) [ICSD 00-030-0877], or the possible products of
the InPW decomposition, like InPO4·2H2O [ICSD 04-009-3660] or
In6WO12 [ICSD 01-0373-5973]. InPW is monoclinic and belongs to
the space group P21/c (Table 1).
2.2. X-ray diffraction patterns
X-ray diffraction patterns were acquired on a Siemens D5005 diffrac-
tometer with Cu Kα radiation, at 40 kV and 40 mA, with a stepsize of
0.02°/1 s. The unit cell parameters were fitted using the Bruker AXS:
TOPAS V2.1 software.
In the 31P MAS NMR spectrum of HPW, two signals can be discerned
at −15.2 and −15.0 ppm, indicating that different counter cations, like
H3O+ and H5O2+, are present in the hydrated material (Fig. 2a). All the
terminal oxygen atoms in the Keggin heteropolyanion are linked via
the equivalent hydrogen bonds with these counter cations [13]. In the
31P MAS NMR spectra of GaPW and InPW, the cations are present prob-
ably as hexaaqua complexes [M(H2O)6]3+, where M=Ga3+, In3+. For
GaPW, two strong signals are seen at −15.0 and −15.6 ppm (Fig. 2b).
The signals in this region are very sensitive toward hydration state of
the sample [14]. Finally, interaction of Keggin units with cations may
bring about formation of lacunar forms or dimers, all these species giv-
ing rise to the signal at −13.5 ppm [13,15]. The signals at −13.4 ppm
of the salts in the present study (Fig. 2b,c) are, however, significantly
weaker than found in AlPW [12].
2.3. Nuclear magnetic resonance spectroscopy
The solid-state NMR experiments were performed on a Bruker
MSL 400 spectrometer at resonance frequencies of 400.3 and
161.9 MHz for 1H and 31P nuclei, respectively. Flip angles of π/2 for
1H and 31P and repetition times of 10 s for 1H and 60 s for 31P nuclei
were used. The 1H and 31P MAS NMR spectra were recorded with a
sample spinning rate of about 10 kHz. The data were processed
with the Bruker software WINNMR and WINFIT.
2.4. Procedure for etherification reactions
The catalytic tests were carried out in a glass, batch reactor work-
ing under atmospheric pressure, and using, unless not specified
otherwise, 5 mol% of a catalyst. Blank tests showed that neither
1-phenylethanol (1-PE) nor styrene (one of the etherification
by-products) reacted with C1–C4 alkanols in the absence of a catalyst.
Before the catalytic studies, the catalysts were dehydrated in vacuum
at 150 °C for 6 h. The substrates, 1-PE and C1–C4 alkanols, were used
in the 1:1 molar ratio, and 5 cm3 of dichloromethane was used as a
solvent. The process was carried out for 4 h at chosen temperature,
sampling the products, and carrying out quantitative analysis by a
Varian CP-3800 gas chromatograph equipped with a FID detector
and the 30 m×0.32 mm DB-WAX capillary column.
Acidic and non-acidic protons can be directly observed via 1H MAS
NMR due to their different chemical shifts, δ1H [16,17]. Thus, a de-
tailed insight into the nature, accessibility, and reactivity of Brønsted
acid sites in various classes of catalysts, including heteropolyacids,
may be accounted for [12,18]. The 1H MAS NMR spectra of the hydrat-
ed and dehydrated GaPW and InPW materials are shown in Figs. 3
and S3 (SM), respectively. At ambient temperature, two signals at
7.6 and 9.2 ppm can be observed for GaPW. The strong signal at
7.6 ppm is superimposed onto a much broader signal, and a signal
at 9.2 ppm occurs as a medium intensity hump. The signal at
7.6 ppm corresponds to H5O2+ groupings (H2O adsorbed on the
Brønsted acid sites) [12,19], while the hump at 6.6 ppm is due to
physisorbed water (Fig. 3a). After dehydration at 120 °C, a single,
symmetrical line and high intensity line appears at 9.2 ppm
(Fig. 3b) assigned to the “free” protons (i.e., strong Brønsted acid
sites). These acid sites are formed during dehydration by dissociative
decomposition of water molecules located on the Ga3+ or (cf. below)
In3+ cations:
3. Results and discussion
3.1. Characterization of the samples
X-ray diffraction patterns of the pristine HPW and its gallium and in-
dium salts are depicted in Fig. 1. Data on parent HPW are included in
SM. The gallium GaPW salt exhibits the strongest reflections at 2θ=
5.9°, 7.8°, 8.8°, 10.3°, 17.9°, 25.4° and 34.7° (Fig. 1b). No other phases
Â
Ã
MðH2OÞ 3þ→ MðOHÞ2þ þ 2Hþ−ðn–2ÞH2O
MðH2OÞn 3þ→ MðOHÞ2þ þ Hþ−ðn–1ÞH2O ; where M ¼ Ga3þ; In3þ
:
Â
Ã
n
Such a mechanism resembles closely formation of protons on
cation-exchanged zeolites [20]. Upon further dehydration, the intensity
of this line remains approximately constant, thus demonstrating the ther-
mal stability of Brønsted acid sites up to at least 250 °C (Fig. 3c). This find-
ing is very important and encouraging from a catalytic standpoint.
Finally, the 1H MAS NMR spectra of InPW reveal generally a similar
behavior (Fig. S3, SM). Upon dehydration at 120 °C, the line at
9.2 ppm, corresponding to the acidic protons, dominates and then ca.
70% of its intensity is lost upon dehydration at 250 °C. Other lines of
protons in the range 3.9–7.6 ppm are also present (Fig. S3 b). We con-
clude that the bare Brønsted acid sites in InPW at 9.2 ppm are much
less stable thermally than in GaPW.
3.2. Etherification of 1-phenylethanol with C1–C4 alkanols
Etherification constitutes an important class of reactions catalyzed
by inorganic and organic acids. The products formed are symmetrical
or unsymmetrical ethers. If, for example, etherification between the
normal alkanols and alkylaromatic alcohols is carried out, then
Fig. 1. X-ray diffraction patterns of the pristine HPW·13-14H2O (a), and the
GaPW·13H2O (b) and InPW·23H2O (c) salts.