P.S. Reddy et al. / Catalysis Communications 11 (2010) 1224–1228
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2. Experimental
porous Zr(MoO4)2 and Zr(WO4)2 compounds in the case of promoted
catalysts as reported earlier [20]. In the XRD patterns of Z, we
observed characteristic peaks at 2θ=28.3, 24.46 and 31.58° corre-
spond to the monoclinic-ZrO2 phase and lines at 2θ=30.27 and
49.21° due to tetragonal-ZrO2 phase (monoclinic-ZrO2 phase domi-
nating over the tetragonal-ZrO2 phase). While the formation of
crystalline ZrTiO4 compound was noted in the case of TZ and
promoted TZ samples. In the case of MTZ and WTZ catalysts, we
also observed the formation of Zr(MoO4)2 and Zr(WO4)2 compounds
respectively, in addition to the ZrTiO4 compound. We did not observe
any peaks due to crystalline MoO3 and WO3 which suggest that a
strong interaction exists between the dispersed MoOx or WOx and the
support. The number of acidic sites of the pure support normally
changes up on incorporation of promoters. In case of WTZ and MTZ
catalysts, the concentration of acidic sites increased significantly
when compared to that of pure TZ support. The number of acidic sites
on the WTZ catalyst are higher than that of the MTZ catalyst.
2.1. Preparation of catalysts
ZrO2 was prepared from an aqueous solution of ZrOCl2 by adding
dilute NH4OH drop-wise up to pH 8. The obtained precipitate was
washed several times with deionised water until free from chloride ions,
dried at 393 K for 12 h and calcined at 923 K for 5 h. The TiO2–ZrO2 (1:1
mole ratio based on oxides), WO3/TiO2–ZrO2 and MoO3/TiO2–ZrO2
catalysts (10 wt.% WO3 or MoO3) were prepared by using coprecipita-
tion and impregnation methods as described elsewhere [20]. The
resulting samples were oven dried at 393 K for 12 h and calcined at
923 K for 5 h in air atmosphere. The investigated ZrO2, TiO2–ZrO2, WOx/
TiO2–ZrO2 and MoOx/TiO2–ZrO2 catalysts are referred to as Z, TZ, WTZ
and MTZ, respectively.
2.2. Catalyst characterization
The Raman spectra of Z, TZ, WTZ and MTZ catalysts are shown in
Fig. 1. The Raman spectra of Z sample exhibited bands corresponding
X-ray powder diffraction patterns have been recorded on a
Siemens D-5000 instrument by using Cu Kα radiation source and
scintillation counter detector. The specific surface areas of the samples
were determined on a Micromeritics Gemini 2360 instrument by N2
physisorption at liquid nitrogen temperature. The NH3-temperature-
programmed desorption (TPD) measurements were performed on an
AutoChem 2910 instrument (Micromeritics, USA) [20]. Raman spectra
were obtained on a DILOR XY spectrometer equipped with a CCD
detector.
to a mixture of monoclinic-ZrO2 (180, 188, 331, 380, and 476 cm−1
)
and tetragonal-ZrO2 (290, 311, 454 and 647 cm−1) phases [22] and
the monoclinic ZrO2 phase dominating over the tetragonal ZrO2 phase
as observed from XRD measurements. The TZ and promoted TZ
catalysts exhibited Raman bands at 168, 280, 338, 412, 640 and
803 cm−1 which must be attributed to ZrTiO4 compound. Raman
bands at high frequency region (700–1100 m−1) of the WTZ catalyst
are due to the presence of geometrically different WOx species on the
surface of the mixed oxide support [20,23]. These compounds may
obstruct some of the active sites on the catalyst surface. This may be
the reason why WTZ catalyst shows comparatively less activity than
MTZ catalyst. Moreover, absence of bands due to crystalline MoO3 and
WO3 obviously reveal that these have strongly interacted with the
support as observed from XRD study. Nevertheless, the MoO3 and
WO3 promoters enhanced the acidity of TiO2–ZrO2 mixed oxide and
improved catalytic properties for the title reaction.
2.3. Activity measurements
The catalytic activity for acetylation of glycerol was carried at
atmospheric pressure in the temperature range of 313–393 K. In a
typical experiment, 2 g of glycerol and 3.9–7.8 mL of acetic acid were
taken in a 100 mL two necked round bottom flask with 0.1 g of
catalyst. A Dean–Stark trap is attached to this round bottom flask to
remove water from the reaction mixture during the reaction because
water is a byproduct in the glycerol esterification reaction, which
favors the deactivation of catalyst and reversibility of the reaction.
Catalysts were pre-activated at 423 K for 2 h before catalytic runs.
Samples were taken periodically and analysed by GC equipped with
BP-20 (wax) capillary column and a flame ionization detector. The
conversion and product selectivity were calculated as per the
procedure described elsewhere [21]. For time-on-stream measure-
ments, the reaction was performed from 0.5 to 70 h using a fresh
catalyst. For catalyst reusablity, the wet catalyst after separating from
the reaction mixture was used and there was no appreciable change in
the catalytic activity up to 5 recycles.
3.2. Catalytic experiments
The reaction products obtained in the acetylation of glycerol are
monoacetin, diacetin and triacetin. Fig. 2 shows the glycerol conversion
3. Results and discussion
3.1. Catalyst characterization
Table 1 represents the BET surface area, amount of NH3 desorbed
and XRD phases of Z, TZ, WTZ and MTZ catalysts. As can be noted from
this table, the promoted TZ samples exhibit less specific surface area
than Z and TZ catalysts. This may be due to the formation of some non-
Table 1
BET surface area, XRD phases and total acidity of Z, TZ, WTZ and MTZ Catalysts.
S. no.
Catalyst
BET SA
(m2/g)
XRD phases
NH3 desorbed
(mmol/g)
1
2
3
4
Z
TZ
WTZ
MTZ
42
30
14
7
m-ZrO2, t-ZrO2
ZrTiO4
ZrTiO4, Zr(MoO4)2
ZrTiO4, Zr(WO4)2
0.21
0.26
0.70
0.61
Fig. 1. Raman spectra of Z, TZ, WTZ and MTZ catalysts.
Reddy, Padigapati S.
