1
66
Ch. Ramesh Kumar et al. / Applied Catalysis A: General 487 (2014) 165–171
other metal salts such as AlTPA and InTPA. The drawback of GaTPA
is its solubility in polar solvents even after exchange of TPA protons
with Ga ions.
In the present work, a series of Ga exchanged TPA supported
on zirconia catalysts were prepared and characterized by differ-
ent spectroscopic and non spectroscopic techniques to elucidate
their physico-chemical properties. The catalysts were evaluated for
benzylation of anisole with benzyl alcohol. The influence of vari-
ous reaction parameters was also studied to optimize the reaction
conditions.
vacuum pressure of 13.3 Pa to minimize surface contamination.
The XPS instrument was calibrated using Au as the standard. For
energy calibration, the C1s photoelectron line was used. The C1s
binding energy was 285 eV. Charge neutralization of 2 eV was used
to balance the charge-up of the sample. The spectra were deconvo-
luted using Sun Solaris Vision-2 curve resolver. The location and the
full width at half maximum value for the species were determined
using the spectrum of a pure sample. Symmetric Gaussian shapes
were used in all cases. Binding energies for identical samples were,
in general, reproducible within ± 0.1 eV.
3
+
31
P nuclear magnetic resonance (NMR) spectra of solids were
◦
2
. Experimental
recorded in a 400 MHz Bruker spectrometer. A 4.5 s pulse (90 )
was used with repetition time of 5 s between pulses in order to
avoid saturation effects. Spinning rate was 5 kHz. All the measure-
2.1. Preparation of hydrous zirconia
ments were carried out at room temperature using 85% H PO4 as
3
Hydrous zirconia was prepared by hydrolyzing the aqueous
standard reference.
solution of ZrOCl ·8H O with ammonium hydroxide at pH of 10.
2
2
The precipitate was filtered off and thoroughly washed with deion-
ized water several times until the chloride content was negligible.
The precipitate was dried at 120 C for 36 h. The hydrous zirconia
2
.4. General alkylation reaction procedure
◦
The alkylation reaction was carried-out in a 50 ml two-necked
after thorough drying was used as support.
round bottom flask provided with a reflux condenser. In a typical
run, 10 g of anisole and 3.37 g of benzyl alcohol (15:5 molar ratios)
along with 0.75 wt% catalyst was taken in flask. The reaction was
2.2. Preparation of zirconia supported gallium exchanged TPA
catalysts
◦
carried out at a reaction temperature of 120 C. The reaction mix-
ture was withdrawn at different intervals and analyzed by a gas
chromatography equipped with a SE-30 column and flame ioniza-
tion detector. The identification of products was made from GC–MS
analysis.
Preparation of Ga3+ exchanged TPA supported on ZrO2 involves
two steps. In the first step, GaTPA was prepared by exchanging the
3
+
protons of TPA with Ga . The required quantity of TPA dissolved
in minimum amount of distilled water and to this the calculated
amount of aqueous Ga(NO3)3 was added drop wise. The resulting
◦
mixture was aged for 1 h at 80 C. The excess water was evaporated
3. Results and discussion
◦
to dryness. The obtained catalyst mass was oven dried at 120 C for
overnight, calcined at 300 C for 2 h.
◦
3.1. Catalysts characterization
In the second step, GaTPA supported on zirconia were prepared
by impregnation method. The required amount of GaTPA dissolved
in distilled water was added to zirconia with continuous stirring.
The resultant mixture was allowed to stand for 3 h and the excess
water was evaporated on a water bath. The dried catalyst masses
Fig. 1 shows the IR spectra of GaTPA/ZrO2 catalysts. The char-
acteristic bands of heteropoly anion were observed in the range of
700–1100 cm . The characteristic bands of pure TPA/PW12O40
−
1
3−
(Fig. 1h) as shown in the insert of the figure appear at 1081, 962,
910, 796 which corresponds to ꢁas(P–O), ꢁas(W–Ot), ꢁas(W–Oc–W)
and ꢁas(W–Oe–W) vibrations, respectively [22]. In the case of
GaTPA/ZrO2 catalysts three characteristic bands were observed at
◦
were kept overnight for further drying in an air oven at 120 C and
◦
finally calcined at 300 C for 2 h. The catalysts were represented
as x wt% GaTPA/ZrO , where x represents the weight percentage of
2
−
1
−1
GaTPA.
1084, 991 and 896 cm and other band at 796 cm is merged with
the band related to ZrO . The observed characteristic bands of Keg-
2
2.3. Characterization of catalysts
gin ion for GaTPA/ZrO2 catalysts indicate that Keggin ion structure
is stable even after exchanging the protons of TPA with Ga ions. In
3+
The Fourier transform infrared (FT-IR) spectra were recorded on
the case of GaTPA supported on ZrO catalysts, characteristic peaks
2
a Bio-rad Excalibur series spectrometer using KBr disc method.
The nature of the acid sites (Bronsted and Lewis) of the cat-
alysts was determined by FT-IR spectroscopy with chemisorbed
pyridine. The pyridine adsorption studies were carried out in the
diffuse reflectance infrared Fourier transform (DRIFT) mode. Prior
to the pyridine adsorption catalysts were degassed under vacuum
of Keggin ion were not observed below 10 wt% loading of GaTPA due
to strong absorption of zirconia in the same region. With increase
in GaTPA loading from 10 to 25 wt% characteristic peaks of Keggin
ion were clearly seen. These results indicate the retention of Keggin
ion structure of GaTPA on zirconia.
GaTPA/ZrO catalysts along with zirconia were characterized by
2
◦
3+
at 200 C for 3 h followed by suspending dry pyridine. Then, the
X-ray diffraction (figure not shown). Ga exchanged TPA showed
◦
excess pyridine was removed by heating the sample at 120 C for
similar diffraction peaks related to that of Keggin ion of TPA. The
◦ ◦ ◦ ◦ ◦
main diffraction peaks were noticed at 7.7 , 8.8 , 10.2 , 14.5 , 17.8 ,
1
h. After cooling the sample to room temperature, FT-IR spectra of
◦
◦
◦
◦
the pyridine-adsorbed samples were recorded.
X-ray powder diffraction patterns were recorded on a Rigaku
Miniflex diffractometer using Cu K␣ radiation (1.5406 A˚ ) at 40 kV
20.5 , 23.1 , 25.3 and 37.5 correspond to monoclinic phase of
GaTPA, which were coincided with those reported in the liter-
ature [21]. No diffraction peaks related to Ga(NO ) ·8H O and
3
3
2
and 30 mA and secondary graphite monochromatic. The measure-
Ga O were observed. This indicates that the protons of TPA were
2 3
exchanged with Ga ions. These results suggest that retention of
Keggin ion structure in Ga exchanged TPA catalyst. The support
◦
3+
ments were obtained in steps of 0.045 with count times of 0.5 s, in
◦
3+
the 2Â range of 10–80 .
XPS measurements were conducted on a KRATOS AXIS 165
with a DUAL anode (Mg and Al) apparatus using an MgK␣ anode.
The non-monochromatized AlK␣ X-ray source (hꢀ = 1486.6 eV) was
operated at 12.5 kV and 16 mA. Before acquisition of the data, each
zirconia showed amorphous behaviour. No crystalline phases were
observed for the GaTPA/ZrO samples. XRD patterns related to Keg-
2
gin ion structure of heteropoly acid and it decomposition products
were absent in all the catalysts. However, peaks related to Keggin
ion were clearly seen in IR spectra of GaTPA/ZrO2 catalysts which
◦
sample was out gassed for approximately 3 h at 100 C under a