Structural and catalytic activity of V2O5-supported on AlPO4 catalysts

Article abstract of DOI:10.1007/s00706-003-0081-6

A series of AlPO₄-V₂O₅ (APV) systems with various vanadia amounts 1-30 mol% were prepared by the impregnation method and calcinated at 400 and 600°C for 4 h. The catalysts were characterized by TG/DTG, DSC, IR spectroscopy, XRD, N₂ adsorption, and electrical conductivity measurements. The surface acidity and basicity of the catalysts were studied by the dehydration-dehydrogenation of isopropyl alcohol and the adsorption of pyridine. The catalytic gas phase esterification of acetic acid with ethyl alcohol was carried out at 210°C in a flow system at 1 atm using air as a carrier gas. The results showed that the catalysts calcinated at 400°C were active and selective towards the formation of ethyl acetate whereas the calcination of samples at 600°C led to a drastic reduction in both activity and selectivity. Good correlations were obtained between catalytic activities towards ester formation and acidity of the prepared catalysts. Springer-Verlag 2004.

Full text of DOI:10.1007/s00706-003-0081-6

Monatshefte f €u r Chemie 135, 357–370 (2004)
DOI 10.1007/s00706-003-0081-6
Structural and Catalytic Activity
of V O -Supported on AlPO Catalysts
2
5
4
1
1;ꢀ
Mohamed M. M. Abd El-Wahab , Abd El-Aziz A. Said
2
,
and Shar S. Al-Shihry
1
Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt
Chemistry Department, Faculty of Science, King Faisal University, P.O. Box 1759,
Al-Hofof 31982, Saudi Arabia
2
Received March 21, 2003; accepted (revised) June 25, 2003
Published online February 5, 2004 # Springer-Verlag 2004
Summary. A series of AlPO -V O (APV) systems with various vanadia amounts 1–30 mol% were
4
2 5
ꢁ
prepared by the impregnation method and calcinated at 400 and 600 C for 4 h. The catalysts were
characterized by TG=DTG, DSC, IR spectroscopy, XRD, N adsorption, and electrical conductivity
2
measurements. The surface acidity and basicity of the catalysts were studied by the dehydration-
dehydrogenation of isopropyl alcohol and the adsorption of pyridine. The catalytic gas phase ester-
ꢁ
ification of acetic acid with ethyl alcohol was carried out at 210 C in a flow system at 1 atm using air as
ꢁ
a carrier gas. The results showed that the catalysts calcinated at 400 C were active and selective
ꢁ
towards the formation of ethyl acetate whereas the calcination of samples at 600 C led to a drastic
reduction in both activity and selectivity. Good correlations were obtained between catalytic activities
towards ester formation and acidity of the prepared catalysts.
Keywords. V O ; AlPO ; Structure; Texture; Catalysis; Esterification.
2
5
4
Introduction
Aluminum phosphate can be used as a catalyst and support. As a catalyst, AlPO is
4
known to be active in several chemical processes such as dehydration, isomeriza-
tion, alkylation, condensation, and Diels Alder cycloaddition [1–6]. Moreover,
AlPO is also used as a support for polymerization, oxidation, hydrogenation,
4
reductive cleavage, or hydration catalysts [1, 7–10]. Now, solid acid-base catalysis
is one of the economically and ecologically important fields in catalysis. The acid-
base properties of AlPO play an important role in catalytic reactions. By modify-
4
ing the acid-base properties of AlPO , the catalytic activity can be controlled. The
4
modification with a metal oxide [1, 10–12], brings about changes in the physico-
chemical properties and catalytic activities of AlPO . Experimental studies of such
4
ꢀ Corresponding author. E-mail: a.a.said@acc.aun.edu.eg

3
58
M. M. M. Abd El-Wahab et al.
systems indicated that coupled techniques can be used to characterize the different
types of bulk and surface active phases resulting from the solid–solid interaction
between V O and AlPO . Moreover, various vanadium phosphates have been
2
5
4
detected in the VPO catalysts, depending on the methods of preparation and con-
ditioning of the conventional catalyst precursor. The current literature disagrees on
the nature of active phase [13–17]. Thus, the activity of AlPO -vanadia catalysts
4
were considerably affected by the chemical and physical nature of the phases
formed during heat treatment of start materials. However, synthesis and physico-
chemical properties of vanadium aluminum phosphate were documented in litera-
ture on the VPO pure compounds [18–21]. On the other hand there are few reports
that studied the formation and physico-chemical properties of the VPO compounds
resulting from the solid–solid interaction between vanadia and AlPO support
4
during heat treatments of the precursor. Therefore, the present work was devoted
to study the physico-chemical properties of the phases which were formed by the
solid–solid interaction between V O and AlPO during heat treatment and their
2
5
4
activities towards the esterification of acetic acid with ethyl alcohol. For this pur-
pose the resulting solids were characterized by DTG, DSC, X-ray diffraction, IR
spectroscopy, surface area, porosity assessment, and surface acidity. We also stud-
ied the catalytic esterification of acetic acid with ethanol in order to understand its
correlation with acidic properties.
Results and Discussion
Catalyst Characteristics
Thermal Analysis
DTG curves of pure AP and mixed with AMV are shown in Fig. 1. The DTG
analysis of pure AP (curve a) shows that AP looses weight with heating over
ꢁ
two stages. The first stage that becomes evident is at 100 C and corresponds to
ꢁ
the desorption of physisorbed water. The second stage occurs at 140 C and is due
to the loss of strongly adsorbed water. Curves b–e represent the DTG curves of AP
which was supported with different ratios of AMV. The analysis of the data of these
curves indicate some trends and the following conclusions may be drawn: (i) The
ꢁ
weight loss on heating up to 200 C exhibits two peaks similar to that of pure AP.
ii) On addition of AMV, new peaks start to appear reaching maxima at the addition
(
of 30 mol%. The maxima of these peaks at 190, 200, 307, and 330 C correspond to
ꢁ
the decomposition of AMV via intermediates into V O [22]. On the other hand, it
2
5
is important to mention here that no weight loss was observed on increasing the
ꢁ
ꢁ
heating temperature above 330 C up to 600 C. Figure 2 represents the DSC results
of pure AP and AP mixed with AMV. Curve a shows that AP exhibits one broad
ꢁ
ꢁ
endothermic peak at about 130 C. The endothermic behavior below 200 C illus-
trates the well known weight loss due to the removal of crystalline water. Curves
ꢁ
b–e for the supported samples show new endothermic peaks between 200 and
ꢁ
500 C. These new peaks may be attributed to the decomposition of AMV into V O
below 350 C, the interaction between V O and AlPO to produce a new phase,
2
5
ꢁ
2
5
4
ꢁ
and=or phase transformation above 350 C.

V O -Supported on AlPO Catalysts
2
359
5
4
Fig. 1. DTG curves of pure AP (a) and AMV supported on AP: 10% (b), 15% (c), 20% (d), and 30% (e)
Fig. 2. DSC curves of pure AP (a) and AMV supported on AP: 10% (b), 15% (c), 20% (d), and 30% (e)

3
60
M. M. M. Abd El-Wahab et al.
Fig. 3. IR spectra of pure AP (a) and AMV supported on AP: 5% (b), 10% (c), 20% (d), 30% (e), and
ꢁ
pure AMV (f) of the samples calcinated at 400 C
IR Spectra
ꢁ
IR spectra of pure AP and AP mixed with AMV calcinated at 400 and 600 C are
ꢁ
shown in Figs. 3 and 4. The samples calcinated at 400 C, Fig. 3, show that, the
spectrum of pure AP exhibits three absorption bands. The first one, assigned at
ꢂ1
ꢀ
ꢀ¼ 3450 cm , is characteristic of an unbonded surface P–OH group, the second
ꢂ1
band, which appeared at ꢀꢀ ¼ 1628 cm , is due to the deformation vibration of
ꢂ1
water, and the third band in the region between ꢀꢀ ¼ 1200 and 1000 cm , is prob-
ably due to the different internal stretching modes of the PO tetrahedra, AlO
4
4
tetrahedra, and AlO (H O) octahedra [23, 24]. It is worth noting that there are
4
2
no bands observed corresponding to pure V O or new phases resulting from the
2
5
interaction between V O and AlPO . On the other hand the spectra of the supported
2
5
4
ꢁ
samples calcinated at 600 C, Fig. 4, show that the bands due to the water molecules
located in the interlayer space have disappeared. Moreover, new bands appeared at
ꢂ1
ꢀ
3
ꢀ ¼ 960, 710, and 620 cm for the samples of AP mixed with 5, 10, 20, and
0 mol% V O . These new bands may correspond to the formation of a new phase.
2 5
X-Ray Diffraction
ꢁ
X-Ray diffraction patterns of AMV supported on AP calcinated at 400 and 600 C
ꢁ
were carried out. The diffractograms of the samples calcinated at 400 C (not

V O -Supported on AlPO Catalysts
2
361
5
4
Fig. 4. IR spectra of pure AP (a) and AMV supported on AP: 5% (b), 10% (c), 20% (d), 30% (e), and
ꢁ
pure AMV (f) of the samples calcinated at 600 C
shown) show that pure AP exhibits an amorphous structure. Even after addition of
5, 10, and 20 mol% V O , little crystallinity is observed and AP remains amor-
2 5
ꢁ
phous. Moreover, the line which is located at 2ꢁ ¼ 26.58 may correspond to the
formation of a ꢂ-VOPO phase [25]. The little crystallinity of this new phase may
4
explain that absorption bands corresponding to this new phase were undetectable
ꢁ
on the IR spectra of the samples calcinated at 400 C. Furthermore, on increasing
ꢁ
the calcination temperature to 600 C, the tested samples exhibit well crystalline
structures, as shown in Fig. 5. It showed that a ꢂ-VOPO phase had developed and
4
ꢁ
gave a predominant diffraction line located at 2ꢁ ¼ 26.39 for the samples contain-
ing V O =AlPO at ratios <20 mol%. In addition, the predominant new line
4
2
5
ꢁ
located at 2ꢁ ¼ 21.68 of 20 or 30 mol% V O in AP corresponds to the formation
2
5
00
of a new phase [25], ꢂ -(VO) P O . It is worth mentioning that the calcination of
2
2 7
prepared catalysts was carried out under a static air atmosphere, thus, the formation
of this new phase may have occurred according to the following solid state reaction.
1
2
VOPO !ðVOÞ P O þ O2
4
2
7
2
2
Moreover, in addition to this new phase, the diffraction lines located at 2ꢁ ¼ 29.77
ꢁ
and 35.96 correspond to the existence [22, 26] of AlVO spinel and Al O ,
2 3
4
respectively.

3
62
M. M. M. Abd El-Wahab et al.
Fig. 5. X-Ray diffraction patterns of pure AP (a) and AMV supported on AP: 10% (b), 20% (c), and
ꢁ
3
0% (d) of the samples calcinated at 600 C
Surface Area and Porosity
The adsorption–desorption isotherms of nitrogen were measured on pure AP and
ꢁ
AP mixed with different ratios of AMV calcinated at 400 and 600 C. The specific
ꢁ
surface areas, SBET, of the samples calcinated at 400 C, and the values of the BET
constant, CBET, are shown in Table 1. The results indicate that AP suffered a
continuous decrease in its SBET value by increasing the loading of V O . However,
2
5
the values of CBET show that the samples 1, 5, and 30 are little greater than 200.
This may taken as an indication that these catalysts are accompanied by micro-
pores. On the other hand for the rest of samples, the CBET value is less than 200.
This may be taken as an indication that basically monolayer–multilayer formation
is operative and it is not accompanied by any meaningful micropore filling.
ꢁ
Table 1. SBET and CBET values of AlPO -V O system calcinated at 400 C
4 2 5
Catalyst
AP
1
5
10
15
20
30
V O
2
5
2
SBET=m g
ꢂ1
62.9
127.8
61.8
250.0
53.0
48.4
118.5
48.8
105.0
40.0
90.8
38.6
203.0
2.0
CBET
216.7
63.0

V O -Supported on AlPO Catalysts
2
363
5
4
ꢁ
Table 2. SBET and CBET values of AlPO -V O system calcinated at 600 C
4 2 5
Catalyst
AP
1
5
10
15
20
30
V O
2 5
2
SBET=m g
ꢂ1
17.7
17.6
8.2
14.7
7.0
14.0
7.4
13.2
8.2
18.0
9.9
19.8
11.0
28.9
6.1
CBET
18.5
On the other hand, the surface area and C
ꢁ
values of the samples calcinated at
00 C are given in Table 2.
It shows a drastic reduction of the SBET and CBET values. The pore volume
BET
6
analysis indicates that the materials contain wide mesopores. These pores may
be created by the sintering process which is responsible for such decrease in the
SBET values.
Electrical Conductivity
The electrical conductivity measurements with and without reactant vapors have
ꢁ
ꢁ
been studied in the range 150–300 C on the catalysts, calcinated at 400 C. The
experimental conditions used are similar to those used in catalytic activity runs.
The results indicated that the electrical conductivity value, increases on increasing
the reaction temperature. Plots of log k against 1=T of pure AP and AP mixed with
V O can be fitted to an Arrhenius relationship [27],
2
5
ꢂDEa=RT
k ¼ k exp
o
where k is the electrical conductivity, k is the pre-exponential factor and DE is
o
a
the activation energy for migration of charge carriers. Values of DE obtained by
a
least squares fitting of the data are shown in Fig. 6. It shows the variation of DEa
with the % loading of V O . Curve a shows that without the reaction mixture (air
only), addition of a low amount of V O led to an observable decrease in the value
2
5
2
5
Fig. 6. Variation of the activation energy (DE ) with the loading of V O of the samples calcinated
a
2 5
ꢁ
at 400 C, (a) air, (b) air þ reactants

3
64
M. M. M. Abd El-Wahab et al.
of DE of pure AP followed by a little decrease observed on increasing to a %
a
content of V O . However, according to XRD results, it can be suggested that the
formation of ꢂ-VOPO which contains V ions in the V valence state should
2
5
5þ
4
4
þ
5þ
decrease the V =V ratio and consequently should decrease the electron jumping
on the catalyst surface. On the other hand curve b shows that on admission of the
reaction mixture, (ethanol þ acetic acid þ air), DE of pure AP decreases from
a
1
.48 to 0.96 eV. Moreover, a remarkable increase in DE value is observed on
a
further increase in V O ratio reaching a maximum on addition of 15 mol%, then
a decrease up to the addition of 30 mol% is observed. However, the comparison
2
5
between DE in presence and absence of the reactant mixtures provides a clear and
a
direct evidence for a redox mechanism with consumption of surface lattice oxygen.
4þ
5þ
Such a redox mechanism requires the presence of V ions in equilibrium with V
ions. However, the formation of ꢂ-VOPO spinel is responsible for the presence of
4
5
þ
V
ions as an acceptor site (acid site) for the electron injected by the alcohol
adsorbed on the catalyst surface. Moreover, the concept of acidic and basic sites of
the catalyst surfaces has been reviewed [28, 29] in relation to a perturbational
molecular orbital theory of acid base interactions. It has been shown [28] that
spatial extent of electronic states at the surface, that is, whether they are delo-
calized or localized, is a determining factor in adsorption and chemical reaction
selectivity. On metal oxides, from the structure and composition stand points, the
spatial extent of electronic states may be controlled by the degree of departure from
stoichiometry [30] as influenced by pretreatment conditions and=or foreign ion
additives. Thus, the width of the energy gap is also important in controlling the
redox mechanism, the number of molecules which can be chemisorbed in the
course of a catalytic reaction, and the nature of the chemical bond between
the molecule and the surface. Accordingly, the modification observed in the Fermi
potential Fig. 6 should play a role in the activity and selectivity of the prepared
catalysts.
Surface Acidity
The catalytic dehydration-dehydrogenation of IPA over V O supported on AlPO
4
2
5
is represented in Fig. 7. The results indicate that the reaction products were mostly
propene and scarcely acetone. However, it has been suggested that the IPA dehy-
dration has been used by several authors [28, 31] as a test reaction for determining
the acidity of different catalysts and it proceeds quickly on weak acid sites [26].
Isopropanol decomposition can form either propene and water (dehydration) or
acetone and hydrogen (dehydrogenation). The different characteristics of the spe-
cies present in the solids are modified in their behaviour as catalysts in the IPA
dehydration reaction, which is used as a comparative measure of the catalyst acid-
ity. Thus, it can be observed from Fig. 7 that the % yield of propene on pure AP
indicates that the reaction of IPA is reflecting the fact that only little weak acid sites
exist on the AP surface. Eventually, the introduction of a low amount of vanadia,
1 mol%, into AP practically does not alter its activity, however, larger amounts (3–
30 mol%) lead to an observable increase in its activity and selectivity of propene
formation, the more the greater the V O loading. However, the acid centers, pre-
2
5
sent at the APV surface, have been identified as hydroxyl groups bonded either to

V O -Supported on AlPO Catalysts
2
365
5
4
Fig. 7. Variation of IPA conversion (o) and yields of propene (D) and acetone (&) with the loading
ꢁ
of V O supported on AP of the samples calcinated at 400 C
2
5
Al, P, or V atoms. These hydroxyl groups are most probably responsible for
Br €o nsted acidity [3, 16]. The Lewis acid sites on APV powders are most probably
5þ
linked to the presence of metal cations, V , which are electron acceptors. The
second site is a basic site due to the presence of oxygen. Moreover, the presence of
V–O–V bonds should increase the basicity of the supported catalysts. As regards
the AlPO support, the surface acidity is low although Br €o nsted acid sites are
4
exhibited. However, the acid properties of AlPO can be modified by introducing
4
elements different from Al and P in the framework at different loading. Thus, it is
evident from Fig. 7 that the incorporation of V O to AlPO results in a remarkable
2
5
4
increase in the number of both Br €o nsted and Lewis acid sites.
It is known that the chemisorption of pyridine (PY) was used as a base probe
determine the acidity of the catalysts [29, 32]. The poisoning of the active sites of
the APV20 catalyst in the IPA conversion was performed through the saturation of
the acid sites with PY according to the following procedure. After measuring the
ꢁ
conversion activity of the APV20 catalyst at 210 C the catalyst was injected with
different volumes of PY in the stream of the reactants using N as a carrier gas. The
2
obtained results represented in Fig. 8 indicate that the PY strongly suppressed the
activity for the IPA dehydration. So, the chemisorbed PY decreases the yield of
the dehydration process from 82 to 20%. The dehydrogenation activity practically
does not change by base poisoning. A drop in the dehydration activity is accom-
panied by a change in product selectivity towards the propene yield, getting greater
as the poisoning effect increases (Fig. 7). The above results demonstrate that in the

3
66
M. M. M. Abd El-Wahab et al.
Fig. 8. Variation of IPA conversion (o) and yields of propene (D) and acetone (&) with volume of
pyridine for the APV20 catalyst
investigated catalysts weak, intermediate, and strong acidic sites are operative. The
admission of PY poisons only the intermediate and strong ones.
Catalytic Activity
The catalytic esterification of acetic acid with ethanol over the catalysts calcinated
ꢁ
ꢁ
at 400 and 600 C for 4 h, was carried out at 210 C. The reaction conditions were:
.5 g of catalyst, 2.1% acetic acid, and 1.6% ethanol reactants were fed into the
reactor after air had been bubbled through thermostated acetic acid and ethanol at a
0
3
ꢂ1
total flow rate of 130 cm min . The analysis of the gas mixture after the reaction
revealed that ethyl acetate and acetaldehyde were the only products. The experi-
mental results of the catalysts calcinated at 400 C are presented in Fig. 9. It shows
that pure AP support is active and selective towards the formation of ethyl acetate.
ꢁ
Moreover, the introduction of vanadia (1–5 mol%) into AlPO steadily increases its
4
activity towards the ester formation. Larger amounts (7–30 mol%) lead to an obser-
vable increase in both activity and yield of ester. This effect increases as V O
loading increases. The IPA conversion and the propene production depend in a
2
5
similar way on the vanadia loading and it is similar to that acidic characteristic

V O -Supported on AlPO Catalysts
2
367
5
4
Fig. 9. Variation of ethanol conversion (o) and yields of ethyl acetate (D) and acetaldehyde (&) with
the loading of V O supported on AP of the samples calcinated at 400 C
ꢁ
2
5
behavior, see Fig. 7. In addition, the correlation between yield of ethyl acetate and
loading of V O shows a straight line, Fig. 10. There seems to be is a linear
2
5
relationship between the catalytic activity to ester formation and the number of
created active acidic sites due to the addition of vanadia into AP.
It is reasonable to mention here that although AP is active and selective towards
ethyl acetate formation, it exhibits low activity to the formation of propene via IPA
decomposition. This means that the AP surface possesses intermediate strength or
strong acid sites which are comfortable sites for the esterification reaction. Thus,
the catalytic activity of AP may be affected by two factors, the chemical nature of
the surface (number and strength of acid sites) and the texture of the solid. Con-
sequently, it appears that the addition of V O (acid catalyst) may activate the sites
2
5
present on the AlPO surface and=or creates new acid sites. The new acid sites can
4
be formed via the creation of V–OH Br €o nsted acid sites close to active centers [33]
00
and the formation of new phases, ꢂ-VOPO and ꢂ -(VO) P O . Moreover, during
4
2 2 7
the reaction, ꢂ-VOPO is slowly converted to vanadyl pyrophosphate in a reaction
4
mixture in which vanadium ions are in the V4 state. In addition, the formation of
þ
the above new phases accompanies the formation of Al O , as indicated by XRD.
2
3
The Al O may form some kind of solid solution and=or a new phase [34], AlVO ,
2
3
4
4
þ
with the excess V O in the solid matrix. The creation of V by the AlVO and
5
ꢂ -(VO) P O spinels should increase the ratio of V =V . Moreover, from the
2
2
4
00
4þ
5þ
2 7
electrical conductivity results, the observed higher degree of conversion as well as
the yield of ester for the samples containing V O (>5 mol%). This may either
2
5
4
þ
5þ
result from the optimum ratio of V =V cations or could also be related to the

3
68
M. M. M. Abd El-Wahab et al.
Fig. 10. Variation of yield of ethyl acetate with loading of V O supported on AP of the samples
2
5
ꢁ
calcinated at 400 C
value of Fermi potential. Thus, the increase in the energy gap of AlPO by the
4
addition of V O during the reaction course may enhance the electron exchange
2
5
between the reactants and the catalyst surface.
In addition, the porosity framework of AlPO can be modified via substituting
4
aluminum or phosphorus by metal atoms in different valence states [35]. Conse-
quently, it is interesting to note that the samples which have the smallest pore
aperture are more active towards esterification. This observation suggests that
the reaction under study is not limited by internal diffusion in the APV samples.
Finally, it is quite unexpected that the esterification selectivity of samples
ꢁ
calcinated at 600 C (results not shown) is decreases drastically, even though the
samples contain ꢂ-(VO) P O as the predominant crystalline phase. This behavior
2
2 7
may be attributed to the drastic decrease in the surface area. It is also possible that
4þ
00
5þ
the formation of ꢂ -(VO) P O is responsible for a reduction of the V =V ratio.
2
2 7
Moreover, the removal of OH groups (i.e. Br €o nsted acid sites), caused by the
ꢁ
calcination of the catalysts at 600 C, Fig. 4, also led to such a decrease.
Conclusions
The modification of AlPO with V O leads to an improvement in the textural
4
2 5
properties and the catalytic activity. The AP treated with V O resulted in an
2
5
enhancement of the dehydration reaction towards the ethyl acetate formation via
the intermediate or strong acid sites. Moreover, the increase of the energy gap of
AlPO by the addition of V O (>5 mol%) during the reaction may enhance the
4
2 5

V O -Supported on AlPO Catalysts
2
369
5
4
electron exchange between the reactants and the acid-base sites on the catalyst
surfaces.
Experimental
Materials
The starting materials AlPO , AP, (Merck) and NH VO , AMV, (BDH) were used for the preparation of
4
4
3
the catalysts. The mixed samples were prepared by impregnation of AlPO with different proportions
4
of AMV dissolved in doubly-distilled water. Yellow mixtures were formed which denoted the formation
ꢁ
of a new phase [36]. The samples produced were dried in an oven at 100 C for 24h before being
ꢁ
calcinated at 400 and 600 C for 4 h in a static air atmosphere. The content of V O added, was varied
2
5
between 1 to 30mol%.
Apparatus and Techniques
Thermal Analysis
A thermal analyzer 2000 TA instrument (USA) controlling a 2050 thermogravimetric (DTG) analyzer
and a 2010 differential scanning calorimeter (DSC) was used. DTG curves were recorded upon heating
ꢁ
ꢂ1
3
up to 600 C at 10 K min and a 30 cm min flow of nitrogen gas. For DSC measurements, a sample
ꢂ1
ꢁ
ꢂ1
3
was heated up to 500 C at 10K min and a flow of 30 cm min of nitrogen gas.
ꢂ1
IR Spectroscopy
ꢁ
ꢂ1
Spectra of the samples calcinated at 400 and 600 C for 4 h were recorded in the ꢀꢀ ¼ 4000–400 cm
region with a Shimadzu spectrophotometer (model 740) using the KBr disc technique.
X-Ray Diffraction
XRD of the test samples was performed with a Philips diffractometer (Model PW 2103) equipped with
˚
Ni-filtered CuK radiation (ꢄ ¼ 1.5418 A, 23kV, and 20mA).
ꢃ
Nitrogen Gas Adsorption
ꢁ
Nitrogen gas adsorption–desorption isotherms were measured at ꢂ196 C using a model ASAP 2010
instrument (Micromeritics Instrument Corporation, USA). Test samples were thoroughly outgassed for
ꢁ
2
h at 200 C. The specific surface area, SBET, was calculated applying the BET equation [R1].
Electrical Conductivity
The electrical conductivity measurements were carried out using a conductivity cell described pre-
viously [37].
Catalytic Activity Measurements
The activity of catalysts towards the esterification of acetic acid with ethanol in the gas phase was
determined in a conventional fixed-bed flow type reactor. The gases after reaction were chromatogra-
phically analyzed by a FID on a Unicam proGC using 2 m DNP and 10% PEG 400 glass columns for
the analysis of the reaction products of ethanol, acetic acid, and isopropanol on the tested catalysts.

3
70
M. M. M. Abd El-Wahab et al.: V O -Supported on AlPO Catalysts
2 5 4
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