W. Ciptonugroho et al. / Journal of Catalysis 340 (2016) 17–29
19
2.2.5. Diffuse Reflectance Infra-Red Fourier Transform (DRIFT) for
pyridine adsorption
illustrates the influence of the WO3 loading on the structural prop-
erties of catalysts calcined at 800 °C. All physisorption isotherms
correspond to type IV isotherms pointing to mesoporous materials.
Incorporating WO3 during synthesis significantly changes the iso-
therm profiles. Upon WO3 addition, a considerable shift of the hys-
teresis to lower partial pressures occurs implying smaller
mesopore diameters. Additionally, the total nitrogen uptake
slightly increases along with increasing WO3 incorporation up to
loadings of around 15 wt.%. Analysing the pore size distribution
in the absence of WO3 emphasises a broad distribution, ranging
from 6.5 to 16 nm (Fig. 2a) together with a low pore volume of
0.05 cm3/g. Upon the addition of 5 wt.% WO3, the pore size distri-
bution becomes narrower with pore diameters of 5–11 nm, respec-
tively. This trend continues for a further addition of WO3 (10–
20 wt.%) resulting in materials of narrower pore size distribution
with maxima located between 5 and 6 nm. As the WO3 content
exceeds 20 wt.%, again a broadening of the pore size distribution
occurs. Along with the described trends, the specific surface areas
of the materials change. For bare mesoporous ZrO2, a specific sur-
face area of only 15 m2/g could be observed together with a grad-
ual increase to 32 m2/g for 5 wt.% WO3 up to 54 m2/g for 15 wt.%
WO3 loading, respectively. At higher loadings of 20–30 wt.% WO3
the specific surface areas remain around 55 m2/g. The presence
of WO3 is associated with the formation of rather stable WAOAZr
bonds on the material surface. This interaction has been described
to reduce the mobility of Zr atoms on the surface potentially sup-
pressing sintering [32].
In addition to WO3 loading, calcination temperature strongly
influences the final textural properties of the catalysts. Selecting
20 wt.% WO3 loading, minor changes of the textural properties
occur for a calcination temperature of 600–700 °C, while a calcina-
tion at 800–900 °C causes a pronounced loss of specific surface
area (Table 1 and Fig. 1c). Calcination at those temperatures also
progressively suppresses the N2 uptake and the hysteresis loops
shift to higher partial pressure. Additionally, the effect of calcina-
tion temperature of catalysts with 20 wt.% WO3 on the pore size
distribution is displayed in Fig. 2b. A temperature increase from
600 to 700 °C results in comparable and narrow pore size distribu-
tions with maxima around 4.6 nm. Meanwhile, calcination at
800 °C leads to broader pore size distributions and lower distribu-
tion intensity centred at 6 nm. The distribution becomes even
broader for calcination at 900 °C. At this temperature, the pore
structure completely collapses forming a broad range of pore sizes
with low distribution intensity. Concerning the surface area, cata-
lysts calcined at 600 and 700 °C reflect comparable surface areas of
111 and 90 m2/g. As the calcination temperature rises to 800 and
900 °C, the obtained specific surface areas decrease to 54 and
25 m2/g, respectively. Calcination at significantly high temperature
favours the mobility of Zr atoms, which promotes sintering accom-
panied by the enlargement of pore size. Hwang et al. observed that
increased calcination temperatures led to a pore size enlargement
of Al–WO3/meso-ZrO2 prepared by a hydrothermal method [10].
Higher calcination temperatures for WO3/ZrO2 prepared from
aerogel and incipient wetness impregnation led to bigger meso-
pores [33].
DRIFT-pyridine measurements were performed by Vertex-70
from BRUKER. The catalyst was firstly preheated to 400 °C under
N2 flow for 30 min in order to remove impurities adsorbed on
the surface. The preheated catalyst was then cooled to 35 °C at
which the pyridine adsorption was conducted. Pyridine adsorption
was carried out by bubbling a N2 flow through liquid pyridine. The
pyridine containing N2 flow was led via the sample. Saturation
with pyridine was performed for 15–20 min. The adsorbed pyri-
dine was then partially desorbed by heating the sample to 150 °C
and holding for 20 min. Prior to recording of the spectra, the sam-
ple was firstly cooled down to 35 °C.
2.2.6. Scanning Transmission Electron Microscopy (STEM)
Scanning transition electron microscopy (STEM) images were
recorded on Hitachi HD2700 microscope. The acceleration voltage
was adjusted to 200 kV. Before measurement, the sample was pre-
pared by scooping dry powder with Cu/lacey carbon grid. The
images were produced from SE, ZC, and TEM detectors.
2.3. Catalysis
LA and 1-butanol (1-BuOH) together with the catalyst (10 wt.%
with respect to LA) were charged into a 100 ml two neck round bot-
tom flask and heated in an oil bath. A vigorous stirring at 750 rpm
was applied during reaction. Sampling was performed withdrawing
approximately 50 mg liquid product and quenching into an ice bath.
Afterwards, the product sample was diluted with 1,4-dioxane con-
taining 15 mg/g diethylene glycol dimethyl ether (DGDME) as an
internal standard before further analysis. The GC quantification was
carried out on Hewlett Packard 6890 series equipped with a FID
detector and a CP-Wax52 column (60 m ꢁ 250
lm ꢁ 0.5
lm). The
sample was carried by a N2 flow with 1.5 ml/min and heated from
50 to 200 °C with a ramping rate of 8 °C/min. The only observed
products were pseudo- (p-BL) and normal-butyl levulinate (n-BL). In
the GC-chromatogram, the peak associated with LA exhibits a distinct
tailing causing less accurate quantification resulting in slight under-
estimation of conversion. Therefore, the presented discussion is
based on the observed yields of p- and n-BL. No other products were
observed in any of these reactions. Therefore the selectivity of n-BL
(Sn-BL) is calculated according to the following equation:
Yn-BL
Yn-BL þ Yp-BL
Sn-BL
¼
where Yn-BL and Yp-BL stand for the yield of n-BL and p-BL, respec-
tively. Furthermore, the turnover frequency (TOF) is based on the
formation of products. As the reaction can even proceed in the
absence of the catalyst, the total yield is firstly normalised to the
obtained yields form the blank test at corresponding reaction times.
Then the TOF calculation is expressed as follows:
À
Á
D
D
YT
t
ꢁ CEst
TOF ¼
C
WO3
(DYT/Dt), CEst and CWO are assigned to the normalised rate of esters
3
Moreover, the dispersion of W atoms over the ZrO2 support is
also determined. In order to account for W-dispersion, the theoret-
ical tungsten surface density (W-SD) is employed which expresses
the number of W atoms covering 1 nm2 area of the catalyst. Table 1
summarises the resulting W-SD as a function of WO3 loading and
calcination temperature. It can be seen that with higher WO3 load-
ings, W-SD increases gradually. Elevating the calcination tempera-
ture from 600 to 700 °C, W-SD increases from 3.6 to 4.6 W/nm2.
Further, exposing the catalyst to 800 and 900 °C gives rise to a sig-
nificant increase of W-SD to 7.7 and 11.7 W/nm2 due to a substan-
tial drop in the specific surface area. Compared to several reports,
production, total molar ester concentration per gram of product and
WO3 molar concentration per gram of catalyst respectively. For fur-
ther information, the detailed calculation results can be found in the
electronic supporting information (ESI).
3. Results and discussion
3.1. Textural properties
The physicochemical properties of the prepared catalysts were
examined by N2 physisorption (Fig. 1 and Table 1). Fig. 1a and b