Selective Oxidation of Methanol to Formaldehyde over Active Molybdenum Oxide Supported on Hydroxyapatite Catalysts

Article abstract of DOI:10.1007/s10562-015-1624-2

Hydroxyapatite (HAP) was synthesized by sol-gel method. Different ratios of molybdenum oxide (1-15 % w/w) supported on HAP were prepared by the impregnation method and calcined at 400 °C in a static air atmosphere. The catalysts were characterized by thermogravimetry, differential thermal analysis, X-ray diffraction, FTIR spectroscopy and nitrogen sorption measurements. The surface acidity of the catalysts was investigated by the dehydration of isopropyl alcohol and the adsorption of pyridine (PY) and 2,6-dimethyl pyridine (DMPY). The gas-phase oxidation of methanol to formaldehyde was carried out in a conventional fixed-bed flow type reactor using N₂ as a carrier gas. The obtained results clearly revealed that HAP-MoO₃ systems were active and selective towards the formation of formaldehyde. The maximum yield of formaldehyde (97 %) was achieved on the catalyst containing 5 wt% MoO₃/HAP. The generation of Mo⁶⁺as Lewis together with Bronsted acid sites play the main role in the formation of formaldehyde. Graphical Abstract: Catalytic oxidation of methanol over MoO₃/HAP calcined at 400 °C for 4 h.

Full text of DOI:10.1007/s10562-015-1624-2

Catal Lett
DOI 10.1007/s10562-015-1624-2
Selective Oxidation of Methanol to Formaldehyde Over Active
Molybdenum Oxide Supported on Hydroxyapatite Catalysts
1
1
1
Abd El-Aziz A. Said
•
Mohamed M. M. Abd El-Wahab
•
Alian M. Alian
Received: 26 July 2015 / Accepted: 2 September 2015
Ó Springer Science+Business Media New York 2015
Abstract Hydroxyapatite (HAP) was synthesized by sol–
gel method. Different ratios of molybdenum oxide
100
(
1–15 % w/w) supported on HAP were prepared by the
8
6
4
2
0
0
0
0
impregnation method and calcined at 400 °C in a static air
atmosphere. The catalysts were characterized by thermo-
gravimetry, differential thermal analysis, X-ray diffraction,
FTIR spectroscopy and nitrogen sorption measurements.
The surface acidity of the catalysts was investigated by the
dehydration of isopropyl alcohol and the adsorption of
pyridine (PY) and 2,6-dimethyl pyridine (DMPY). The
gas–phase oxidation of methanol to formaldehyde was
carried out in a conventional fixed-bed flow type reactor
Coversion
Y. of Formaldehyde
Y. of Formic acid
using N as a carrier gas. The obtained results clearly
2
0
revealed that HAP–MoO systems were active and selec-
3
nHAP
1
3
%
5
7
10
15
tive towards the formation of formaldehyde. The maximum
yield of formaldehyde (97 %) was achieved on the catalyst
Mixing of MoO3
6
containing 5 wt% MoO /HAP. The generation of Mo as
?
3
Lewis together with Brønsted acid sites play the main role
in the formation of formaldehyde.
Keywords MoO Á Hydroxyapatite Á Surface area Á
3
Graphical Abstract Catalytic oxidation of methanol over
Acidity Á Oxidation Á Methanol
MoO /HAP calcined at 400 °C for 4 h.
3
1
Introduction
Methanol, one of the most important chemical intermedi-
ates, used in chemical industry. It is the starting material
for the synthesis of various products such as hydrocarbons
and oxidation products [1]. Formaldehyde production is the
major one among the oxidation processes. Two processes
are generally used in the industry to produce formaldehyde,
both using methanol as the starting material [2]. The
dehydrogenation of methanol-rich air mixture over silver
catalyst and direct oxidation of methanol-poor air mixture
over iron molybdate catalyst. Both processes are still in use
[3]. The choice between silver and iron molybdate catalysts
&
Abd El-Aziz A. Said
aasaid55@yahoo.com
1
Chemistry Department, Faculty of Science, Assiut
University, Assiut, Egypt
123

A. E.-A. A. Said et al.
must be based not only on economic aspect, but should also
take into account the product end-use, size of plant and the
type of operation [4].
ammonia solution as agent for pH adjustment. A suspen-
sion of 0.24 M Ca(NO ) Á4H O in 350 mL bi-distilled
3
2
2
water was vigorously stirred and its temperature was
Methanol oxidation into formaldehyde occurs selectively
on MoO loaded on such weak basic supports as SnO , Fe O
2 3
maintained at 75 °C. A solution of 0.29 M (NH ) HPO in
4
2
4
250 mL bi-distilled water was slowly added dropwise to
3
2
and ZrO , among which SnO has the most prominent effect
2
Ca(NO ) Á4H O and citric acid solution. In all experi-
2
3 2
2
[
5]. The effect of the support on the activity is an interesting
ments, the pH of the solutions was adjusted to 11. The
hydroxyapatite (HAP) was filtered, washed with bi-dis-
tilled water, dried overnight at 100 °C and calcined in air at
400 °C for 4 h.
chemical aspect to understand the role of support on the
metal oxide. A lot of investigations on the prominent effect
of the support have been reported in the literature [6–8]. It
was concluded that acidity was a key parameter to control the
activity for methanol oxidation [9]. On the other hand
hydroxyapatite (HAP) has attracted the attention of
researchers to the biomaterials field in recent years [10].
HAP is a highly non-staichiometric calcium phosphate
compound with a Ca/P molar ratio ranging from 1.5 to 1.67.
It is also known [11] to have the character of both acidic and a
basic character. Moreover, HAP as support offers high sta-
bility and various substitutions are allowed by the apatite
structure [12]. Various transition metal cations, which have
potential as catalytic active centers, can be readily accom-
modated into the apatite framework based on the large cation
exchange ability of HAP [13–15]. In addition sol–gel syn-
thesis of HAP has attracted much attention [16–19]. This
method offers a molecular-level mixing of the calcium and
phosphorus precursors, which is capable of improving the
chemical homogeneity of the resulting HAP to a significant
extent, in comparison with conventional methods such as
solid state reactions [20], wet precipitation [21] and
hydrothermal synthesis [22].
2.3 Preparation of MoO Supported
3
on Hydroxyapatite Catalysts
The catalysts were prepared by the impregnation method.
Calculated amounts of (NH4)6 Mo O Á4H O (AHM)
7
24
2
(1–15 wt%) dissolved in bi-distilled water were admixed
carefully with calculated amounts of hydroxyapatite. The
samples produced were dried in an oven at 100 °C for 24 h
before being calcined at 400 °C for 4 h in the static air
atmosphere, and then quenched to room temperature.
2.4 Apparatus and Techniques
2.4.1 Thermal Analysis
Thermogravimetry (TG) and differential thermal analysis
(DTA) curves were recorded upon heating up to 700 °C at
-
1
-1
10 °C min and a 40 ml min flow of air atmosphere,
using a computerized Shimadzu Thermal Analyzer TA60
Apparatus (Japan).
However, the catalytic gas phase oxidation of methanol
over MoO supported on nano hydroxyapatite synthesized by
3
sol–gel method to our best knowledge has not been reported
except our recent short work [23]. Therefore, the objective of
2.4.2 X-ray Diffraction
the present study was devoted to the study of MoO supported
3
X-ray diffraction (XRD) analysis of the catalysts was
performed with a Philips (The Netherlands) diffractometer
on hydroxyapatite in detail as an efficient and competitive
catalyst for conversion of methanol to formaldehyde.
˚
(Model PW 2103, k = 1.5418 A, 35 kV and 20 mA) with
a source of CuKa radiation (Ni filtered). An online data
acquisition and handling system facilitated the automatic
JCPDS library search and match for phase identification
purposes. The crystallite size (d), of the synthesized cata-
lysts was calculated using the following Debye–Scherrer
formula [25],
2
Experimental
2
.1 Materials
Methyl alcohol, citric acid, calcium nitrate, diammonium
hydrogen phosphate and ammonium hydroxide were
obtained as pure reagents and were used without further
purification. Ammonium heptamolybdate was supplied
from Merck, (Darmstadt, Germany).
ðdÞ ¼ Kk=B cos h
The shape factor, K, was taken as 0.9, while k for Cu Ka
radiation is 1.5418 A. Where k is the wavelength of the
X-ray (nm), B is the full width of a diffraction peak under
consideration (rad) in the middle of its height that was
considered after computer-fit of the X-ray data using the
Gaussian line, shape, and h is Bragg’s angle, that is
obtained using (1 1 1) line of the pure silicon as the
standard.
˚
2
.2 Synthesis of Hydroxyapatite by Sol–Gel Method
The citric acid sol–gel method [24] using Ca(NO ) Á4H O,
3
2
2
C H O Á4H O and (NH ) HPO as strating materials,
6
8
7
2
4 2
4
1
23

Selective Oxidation of Methanol to Formaldehyde Over Active Molybdenum Oxide Supported…
2
.4.3 FTIR Spectroscopy
methyl alcohol and nitrogen were introduced into the
reactor after nitrogen was bubbled through methyl alcohol
-
1
FT-IR spectra of the prepared catalysts calcined at 400 °C
for 4 h were recorded using a Nicolet spectrophotometer,
model (6700), equipped with data station in the range of
saturator. The total flow rate was fixed at 100 mL min
with 1.6 % reactant of methanol in the gas feed. The
connection between the reactor and the gas chromatograph
(GC) was heated by resistance wire to prevent any con-
densation. The reaction products were chromatographically
analyzed by FID with a Unicam ProGC using a DNP glass
column (2 m). Measurements of the conversion and yield
(%) were recorded after 2 h from the initial introduction of
methyl alcohol into the reactor to ensure the attainment of
the reaction equilibrium, (steady state conditions). The
degree of methanol conversion, selectivity and yield of
formaldehyde are calculated as follows [27]:
-
1
2
000–400 cm with a KBr disc technique.
2
.4.4 Nitrogen Sorption
Nitrogen gas adsorption–desorption isotherms were mea-
sured at -196 °C using a Nova 3200 instrument (Quan-
tachrom Instrument Corporation, USA). Test samples were
thoroughly outgassed for 2 h at 200 °C to a residual
pressure of 10 torr, and the weight of the outgassed
sample was that used in the calculation. The specific sur-
face area, SBET was calculated by applying the Brunauer–
Emmett–Teller (BET) equation. The porosity of the cata-
lysts was determined from the desorption curves using
Nova enhanced data reduction software (Version 2, 13).
-
5
½
CH OHŠ À½CH OHŠ
3
in
3
out
Conversion ð%Þ ¼
Selectivity ð%Þ ¼
 100
 100
½
CH OHŠ
3
in
½
ProductŠ
½CH OHŠ À½CH OHŠ
3
in
3
out
ðConversion  SelectivityÞ
Yield ð%Þ ¼
2
.4.5 Acidity Determination
100
In the previous equations, [CH OH] and [CH OH]
out
3
in
3
The acidity of 10 wt% MoO /HAP catalyst was determined
3
denote the peak areas (concentrations) of methanol at the
reactor inlet and outlet, respectively.
by studying the dehydration of isopropyl alcohol (IPA), the
adsorption of pyridine (PY) and 2,6-dimethyl pyridine
(
DMPY). The dehydration reaction of IPA was carried out
in a conventional fixed bed flow Pyrex glass tube reactor, at
3 Results and Discussion
3.1 Thermal Analysis
atmospheric pressure using N as a carrier gas. The reac-
2
tion conditions were: 500 mg catalyst weight, 2 % reactant
-
of IPA in the gas feed, 100 mL min total flow rate and
1
2
20 °C reaction temperature. The adsorption of PY and
Figure 1 shows the TG and DTA, curves of pure hydrox-
yapatite (HAP), and ammonium heptamolybdate (AHM)
supported on HAP. The TG and DTA curves of pure HAP
exhibit four steps of weight loss that are accompanied with
two endothermic peaks minimized at 130, 185 °C and two
exothermic peaks maximized at, 264 and 445 °C. From
these curves it is evident that, on heating the parent of HAP
up to 200 °C, TG curve is displaying two weight losses.
The first may correspond to the physically adsorbed water
and the second belongs to the removal of the structure
water molecules. On increasing the heating up to 700 °C,
two losses steps with exothermic peaks maximized at 264
and 445 °C. These two steps may be attributed to the
removal of ammonium nitrate from the gel and crystal-
lization of Ca (PO ) (OH) [24, 28–30] respectively.
DMPY were carried out by injection of different volumes
at steady state conditions, according to the previous method
[
26]. The effect of reaction temperature on the activity
variation of presaturated catalyst towards IPA dehydration
was tested. The strength of acid sites was investigated by
DSC measurement. About 10 mg of pyridine-saturated
sample was subjected to DSC analysis. The DSC analysis
was recorded on heating from room temperature up to
-
1
4
00 °C with
a
heating rate of 10 °C min
and
-
0 ml min flow of N , using Auto Q20 DSC with mass
1
3
2
flow apparatus (USA).
.4.6 Catalytic Activity Measurements
2
1
0
4 6
2
The catalytic activity of the catalysts for the vapor-phase
oxidation of methanol was carried out at 400 °C in a
conventional fixed-bed flow type reactor at atmospheric
pressure using dry nitrogen as a carrier gas. A 500 mg of
catalyst was placed in the middle of the reactor with quartz
wool. Space in the reactor pre-and post-heating zone was
filled with glass beads to reduce the effect of auto-oxida-
tion of the substrate and products in the gas phase. A
TG and DTA curves of AHM supported on HAP reveal
that the loss of weights are maximized at 130, 200 and
445 °C as indicated by the DTA curves. The first region of
(30–150 °C) which accompanied with *4.8 % weight loss
with endothermic peak minimized at 130 °C is attributed to
the removal of physically adsorbed water. The second step
is in the range of (150–250 °C) which accompanied with
*43.4 % weight loss with exothermic peak maximized at
123

A. E.-A. A. Said et al.
♦
♦
♦
8
4
0
0
Pure MoO3
♦
♦
♦
♦
1
5% MoO3
♦
♦
♦
♦
•
8
4
0
0
1
0% MoO3
1
5% MoO3
•
•
•
•
•
•
♦
♦
•
8
4
0
0
•
5
% MoO3
•
10% MoO3
•
•
8
4
0
0
pure HAP
600
ο
♦
pure MoO3
0
100
200
300
400
500
700
• CaMoO4
ο Pure nHAP
ο
ο
Pure HAP
o
Temperature ( C)
ο
Fig. 1 TG and DTA curves of pure HAP and AHM supported on
HAP
10
20
30
40
2
50
60
70
Fig. 2 X-ray differaction of pure HAP, pure MoO
supported on HAP calcined at 400 °C for 4 h
3
and MoO
3
2
00 °C due to the removal of structural water and/or
decomposition of organic constituents [31]. The third
region of (400–500 °C) which accompanied with *16.8 %
weight loss with exothermic peak maximized at 475 °C
in agreement with that were reported by Yoon et al. [32].
On the other hand, a small peak detected corresponding to
MoO , start to appear with the addition of 15 wt% MoO .
corresponds to the formation of CaMoO spinel. Moreover,
4
3
3
no weight loss is observed on heating up to 700 °C. This
means that CaMoO formed at lower temperatures is stable
The crystallite size (d), of the synthesized HAP powder
calcined at 400 °C was calculated to be 8.7, 4.9 nm for
planes (210) and (211). Moreover, the average crystallite
size is 6.8 nm. In addition the average crystallite size of
HAP supported 10 wt% MoO3 calcined at 400 °C is
4
spinel [32, 33]. So, our findings were found much satisfied
with the results that reported by Klinkaewnarong et al.[30]
and Eslami et al.[33].
11 nm.
3
.2 X-ray Diffraction Analysis
3.3 Surface Area and Porosity
X-ray diffraction (XRD) patterns of the pure HAP, pure
MoO and MoO supported on HAP calcined at 400 °C
Adsorption–desorption isotherms of nitrogen gas were
measured for the calcined products of AHM supported on
nano hydroxyapatite at 400 °C in the air for 4 h and pre-
sented in Fig. 3. The specific surface area SBET was
obtained by applying the BET equation in its normal range
3
3
was carried out. Phase analysis was done using PDF card
no. 85-0585 for CaMoO , which provide information on
4
the 2h range of 4°–80°. The patterns of the as-synthesized
HAP powder calcined at 400 °C is presented in Fig. 2. It
shows that there are several peaks i.e. (111), (002), (210),
o
of applicability (0.05–0.30 p/p ) with cross-sectional area
˚
= 16.2 A and are represented in Table 1. Figure 3
(
211), (300), (202), (222), (213) and (004) are identical to
of N
2
those of the reference. It is clear from the figure that, the
shows that all isotherms of pure nHAP and loaded catalysts
resulted in type II of Brunauer classification [34]. In
addition, all catalysts exhibit a hysteresis loop, which
closes at lower relative pressures. All hysteresis loops
belong to types E with little of type A of de Boer’s clas-
sification [35]. The results in Table 1 indicate that, the pure
sample of HAP calcined at 400 °C exhibits SBET value of
sample containing MoO with a ratio of (10 wt%) on HAP
3
shows new peaks. These peaks may be attributed to the
formation of a new phase nanocrystalline CaMoO . In
4
addition, all of the prominent peaks corresponding to the
CaMoO phase were appeared clearly without any peaks
4
assigned to either, HAP, CaO and CaCO . These results are
3
1
23

Selective Oxidation of Methanol to Formaldehyde Over Active Molybdenum Oxide Supported…
Figure 5 represents the pore volume distribution curves
for pure HAP and supported with MoO3 calcined at
400 °C. Also, the total pore volume and average pore
diameter were calculated from the desorption curves and
presented in Table 1. It can be seen from this figure that the
pore spectra of pure HAP calcined at 400 °C shows one
4
0
35
HAP
% MoO3
0% MoO3
5
1
30
1
5% MoO3
25
20
-
sharp peak maximized at the value of rp equal to 4.3 nm.
15
This value again indicates that the HAP support exhibits
microporous nature. Upon addition of MoO up to 10 wt%,
3
10
-
the values of r are quite similar to that of pure HAP. On
p
5
0
further increase the content of MoO up to 15 wt%, the
3
mean pore radius increases to 6.4 nm. Moreover, the cal-
culated particle sizes (DBET) of HAP and HAP supported
0.0
0.2
0.4
0.6
0.8
1.0
o
P/P
with 10 wt% MoO calcined at 400 °C were found equal to
3
10 and 12 nm, respectively. These results are in a good
Fig. 3 Nitrogen adsorption–desorption isotherms of pure HAP and
MoO
supported on HAP calcined at 400 °C for 4 h
agreement with that obtained from XRD calculations.
3
3.5 Acidity Determination
2 -1
6.1 m g . The addition of AHM into HAP support led
5
to a continuous decrease in SBET values up to the addition
of 15 wt%. This behavior may be attributed to the forma-
The catalytic dehydration of IPA over 10 wt% MoO3
supported on HAP calcined at 400 °C for 4 h was carried
out and the results are shown in Fig. 6. The results revealed
that the reaction products of IPA was propene (major) with
acetone (minor) not exceed 5 %. However, the reaction of
IPA does not provide information to distinguish between
Lewis and Brønsted acid sites. So, the adsorption of PY
and DMPY as a basic probes are better suited to differ-
entiate between Lewis and Brønsted acid sites [37]. Con-
sequently, the poisoning of the active surface sites of the
catalyst in IPA conversion was performed by saturation of
acid sites with injection of different volumes of PY or
DMPY in the stream of the IPA reactant and the results are
presented in Fig. 6. The results show that the injection of
PY or DMPY led to a continuous decrease in the conver-
sion of the IPA and the yield of propene up to the addition
of 1 ll then a steady state is reached. This behavior reflects
that PY and DMPY decrease the conversion and yield of
propene by 55 and 50 %, respectively. It is clear from this
result that there is a little difference (&5 %) between the
amounts adsorbed from PY and DMPY which is corre-
sponding to the presence of Lewis acid sites. In addition
tion of CaMoO spinal which is exhibits low surface area
4
together with low surface area of pure MoO3.
3
.4 Assessment of Porosity of MoO /HAP Catalysts
3
The assessment of porosity for pure HAP and supported
MoO with (1–15 wt%) calcined at 400 °C was carried out
3
by the application of the V - t plot method and the results
a
are shown in Fig. 4. Also the S values were calculated
t
from the slope of the straight lines of the investigated
catalysts and are cited in Table 1. However, the shape of
the V - t plots may indicate the type of existing porosity.
a
Downward deviation indicates microporosity while upward
deviation indicates the existence of wide pores, mesopores
[
36]. From Fig. 4 it can be seen that, the feature of the
V - t plots of HAP and MoO supported on HAP catalyst
a
3
exhibits a downward deviation up to the addition of
5 wt%, which is indicated microporous nature. These
1
results demonstrate that the formation of new spinel does
not change the micro porosity of HAP support.
Table 1 Texture data of the
MoO /HAP catalysts calcined at
Mixing
%
S
m g
BET
S
t
Total pore
Volume (cc g )
Average pore diameter
(nm)
3
2
-1
2
m g
-1
-1
4
00 °C for 4 h
Pure HAP
56.1
47.3
40.6
35.4
29.0
22.2
8.8
55.2
47.0
40.2
34.3
28.1
22.2
8.8
0.0603
0.0503
0.0417
0.0342
0.0301
0.0254
0.0230
4.3
4.2
4.0
4.1
4.5
4.3
6.4
1
3
5
7
1
1
% MoO
% MoO
% MoO
% MoO
3
3
3
3
0 % MoO
5 % MoO
3
3
123

A. E.-A. A. Said et al.
1
00
0
0
.02
.01
1
5% MoO3
8
0
PY
DMPY
0
0
.00
.02
60
1
0% MoO3
40
0
.01
2
0
0
0
.00
.04
5
% MoO3
0
0.0
0.5
1.0
1.5
2.0
0
.02
Volume of PY and DMPY, μL
0
.00
.06
Fig. 6 Activity variation of IPA with the volume of PY and DMPY
0
over 10 wt% MoO /HAP calcined at 400 °C in a static air atmosphere
3
pure HAP
0.4
for 4 h
0
0
0
.04
.02
.00
Table 2 Activity variation of IPA over unsaturated and saturated 10
3
wt% MoO /HAP with PY and DMPY at different reaction
temperatures
0
.0
0.8
t(nm)
1.2
1.6
2.0
Reaction temp. (°C) % yield of propene
Unsaturated catalyst Saturated catalyst with
Fig. 4 V - t plots of pure HAP and MoO
catalysts calcined at 400 °C for 4 h
a
3
supported on HAP
PY
DMPY
1
1
1
25
50
75
0
0
0
25
51
75
95
95
10
24
52
85
95
15
29
57
90
95
1
0
.4
.7
1
5% MoO 3
200
25
250
0
1
.0
.4
2
0
.7
1
0% MoO 3
&40 % of available acid sites still working to converting
IPA to propene which confirms that, the PY or DMPY
adsorbed on the catalyst surface did not poisoning weak
acid sites. Therefore, from the above results we see that the
distribution of the acid sites of the catalyst under investi-
gation is (&45 %) Brønsted, (&5 %) Lewis and (&40 %)
weak Lewis acid sites.
0
1
.0
.4
0
0
.7
5
% MoO3
.0
2
Moreover, the effect of reaction temperature on the
dehydration of of IPA over 10 wt% MoO supported on
3
1
0
pure HAP
16
HAP previously saturated with PY and DMPY was carried
out and the obtained results are cited in Table 2. The
results reveal that the adsorption of PY or DMPY retards
the conversion activity of IPA more than that of unsatu-
rated catalyst. In addition, the catalyst restored its activity
(acidity) after removal of PY or DMPY on increasing the
reaction temperature up to 225 °C. The retardation of the
0
4
8
12
20
-
Mean pore radius, r (nm)
p
Fig. 5 Pore volume distribution of pure HAP and MoO
on HAP catalysts calcined at 400 °C for 4 h
3
supported
1
23

Selective Oxidation of Methanol to Formaldehyde Over Active Molybdenum Oxide Supported…
catalyst activity may be attributed to the bond strength
between PY or DMPY and the acid sites. DSC curves of
the desorption of PY from presaturated pure nHAP and
gives the relationship between the % conversion and yield
with % mixing of MoO . From this figure, one can observe
3
that pure HAP precalcined at 400 °C exhibits some activity
towards the formaldehyde formation (&8 %) and formic
acid (&17 %). The addition of a little amount of MoO3
(1 wt%) into HAP precalcined at 400 °C shows an
observable increase in both methanol conversion (&73 %)
and yield of formaldehyde (&70 %) while a sharp
decrease in the yield of formic acid (&3 %). In addition,
1
0 % MoO /HAP catalysts are carried out and presented in
3
Fig. 7. The DSC curve of pure HAP shows only an
endothermic peak covered the temperature range from 40
to 103 °C. This peak may be attributed to the removal of
physical adsorbed water and PY desorption from weak acid
sites. On the other hand, the DSC curve of 10 % MoO3/
HAP exhibits endothermic peak extended from 62 to
upon increasing % loading of MoO up to 5 wt% a maxi-
3
2
34 °C which minimized at 108 °C may be attributed to
mum yield of formaldehyde (&97 %) is obtained followed
by a steady state up to the addition of 15 wt%.
the desorption of PY from weak and moderate acid sites. In
addition an exothermic peak extended from 235 to 355 °C
and minimized at 290 °C corresponds to the presence of
moderate acid sites. So, the above results reflect that, the
available acid sites exhibit weak and moderate strengths.
The above results demonstrate that the % loading of
MoO to HAP is playing a main role in the activity and
3
yield of formaldehyde. On the other hand, the catalyst
containing 5 wt% MoO precalcined at 400 °C exhibits
3
excellent activity and selectivity towards the formation of
formaldehyde. These results are very interesting not only
for the high selectivity of the catalyst towards the forma-
tion of formaldehyde, but also in the capacity of the cata-
lyst towards formaldehyde formation which is stable for a
long duration time 90 h (not included).
3
.6 Catalytic Activity
The catalytic dehydrogenation of methyl alcohol over
different percentages loading of MoO supported on HAP
3
catalysts precalcined at 400 °C for 4 h was carried out at
the reaction temperature of 400 °C. The analysis of the gas
mixture after reaction revealed that formaldehyde forma-
tion was the major and formic acid was a minor product,
and the experimental results are presented in Fig. 8. It
On the other hand, formaldehyde is obtained from
oxidative dehydrogenation of methanol by two processes,
(i) on iron–molybdate catalysts and (ii) on silver based
catalysts. In both processes, a conversion of about 99 and
92 % selectivity are obtained. The temperature of the
reaction is 400 °C in the first and about 650 °C in the
second process. In addition, iron–molybdate based cata-
lysts get deactivation through a loss of molybdenum in a
form of volatile compound and the presence of excess
methanol also led to decrease in the activity [38]. More-
over, the silver process has high operating costs. Conse-
quently, the comparison between our present work with the
-
-
-
-
1.0
1.5
2.0
2.5
1
0% MoO /HAP
3
o
2
90 C
1
00
o
1
08 C
8
0
0
Coversion
Y. of Formaldehyde
Y. of Formic acid
pure HAP
-
-
-
-
1.8
2.0
2.2
2.4
6
40
2
0
o
4
3 C
0
50
100
150
200
250
300
350
400
10
nHAP
1
3
%
5
7
15
o
Mixing of MoO3
Temperature ( C)
Fig. 7 DSC curves of PY desorption from pure nHAP and 10 wt%
MoO /HAP presaturated catalysts
Fig. 8 Catalytic oxidation of methanol over MoO
400 °C for 4 h
3
/HAP calcined at
3
123

A. E.-A. A. Said et al.
two commercial processes of methanol oxidation to
formaldehyde, it is remarkable that the yield of formalde-
1
0% MoO /HAP
3
hyde is 97 % over 5 wt% MoO /HAP catalyst. So, our
3
results demonstrate at least two advantages. Firstly, the
high yield of formaldehyde is obtained. Secondly, the
activity of the catalyst is stable for a long duration time
(
90 h).
However, methanol oxidation is a reaction well adapted
to characterize the redox and acidic active species on the
catalyst surface. The structural information at the molec-
ular level about the catalytic sites can be deduced from the
catalytic behavior of this reaction [1]. A selective forma-
tion of methylal ((CH O) CH ) can be assigned to a dual
pure HAP
3
2
2
site, including a redox dehydrogenating site and a Lewis
acid center, whereas the formaldehyde (CH O) formation
2
requires only a redox dehydrogenating site. Methyl formate
(
(
HCOOCH ) can be generated via an oxidation reaction
3
redox centers) followed by the dehydration (on acidic
sites). The surface basic sites are responsible for the
CO ? CO formation. The selective formation of dimethyl
2
ether (CH OCH ) is assigned to the presence of strong acid
3
3
sit. In principle the Brønsted acidity [39, 40] is character-
istic of oxides in which the transition element is in a high
oxidation state, e.g. V O , MoO , CrO , etc. According to
2
000
1800
1600
1400
1200
1000
800
600
400
-
1
Wavenumber (cm )
2
5
3
3
Davydov [39], the Brønsted acidity is responsible for the
selective oxidation of methanol to formaldehyde. Klis-
surski et al. [41] results are in agreement with the above
suggestions. The results of acidity determination Fig. 6,
indicate the creation of Brønsted acid sites with interme-
diate strength existed on the catalyst surfaces as predicted
from Fig. 7 exhibit a higher activity in methanol oxidation
to formaldehyde [42]. This is probably due to the fact that
impregnation ensures better dispersion of molybdenum
trioxide in the HAP support. So, Mo cation selectivity
interacts with hydroxyl groups basic nature and suitable
strength leading to the formation of monolayer patches of
Fig. 9 FTIR spectra of pure HAP and 10 % MoO
calcined at 400 °C
3
/HAP catalysts
3
•
•
.7 Conclusions
HAP was synthesized via sol gel method and loaded
with different ratios of MoO3.
The catalysts calcined at 400 °C revealed that the
formation of CaMoO phase, which is confirmed by
4
XRD and supported by DTA analysis.
•
A continuous decrease in SBET values for all catalysts
upon increasing the % loading of MoO . The assess-
3
MoO support surface. Moreover, catalyst containing dis-
3
ment of the catalysts porosity by V - t plots and mean
a
persed Mo centers on oxide supports are known to have
high acid properties due to the development of molybdate
and poly molybdate species possessing Brønsted and Lewis
acid sites [43, 44]. The HAP surface contains hydroxyl
groups which are weakly acidic, then the deposition of M–
O phase is expected to give rise to a remarkable increase in
acidity. Moreover, in the investigated catalyst containing
pore radius showed a microporous structure of all
catalysts under investigation.
•
•
The acidity determination confirmed that the major
Brønsted and Lewis acid sites existed on the catalyst
surfaces with intermediate strength were responsible
for the higher activity towards formaldehyde formation.
The maximum yield of formaldehyde (97 %) was
achieved on the catalyst containing 5 wt% MoO3.
1
0 wt% MoO , the FTIR spectra, Fig. 9 shows that a band
3
-
2
of Mo–O stretching vibration in MoO tetrahedrons [45]
4
-
1
at 774–905 cm . This result is in agreement with that
reported by Crosman et al. [46] and Wakman et al. [47].
Furthermore, the formation of CaMoO upon calcination of
4
References
MoO supported on HAP with different ratios at 400 °C is
3
6
?
6?
accompanying with Mo . So, the creation of Mo as
Lewis with Brønsted acid sites may be active and play a
main role for methanol oxidation to formaldehyde.
1
. Tatibouet JM (1997) J Appl Catal A 148:213
2. Stiles B, Koch TA (1995) Catalyst manufacture, vol 2. Marcel
Dekker, New York, p 197
1
23

Selective Oxidation of Methanol to Formaldehyde Over Active Molybdenum Oxide Supported…
3
4
. ECN (1994) Process Review, April 30
. Chauvel R, Curty PR, Maux R, Petipas C (1973) Hydrocarb
Process 52:179
. Yang TJ, Lunsford JH (1987) J Catal 103:55
. Matsuoka Y, Niwa M, Murakami Y (1990) J Phys Chem 94:1477
. Kim DS, Wachs IE, Segawa K (1994) J Catal 149:268
. Hu H, Wachs IE (1995) J Phys Chem 99:10911
. Niwa M, Igarashi J (1999) Catal Today 52:71
27. Waterhouse GIN, Bowmaker GA, Metson JB (2004) Appl Catal
A 265:85
28. Ramanan SR, Venkatesh R (2004) Mater Lett 58:3320
29. Han Y, Wang X, Li S, Ma X (2009) J Sol–Gel Sci Technol
49:125
30. Klinkaewnarong J, Swatsitang E, Maensiri S (2009) Solid State
Sci 11:1023
31. Ryu J, Yoon J, Lim C, Oh W, Shim K (2005) J Alloys Compd
390:245
5
6
7
8
9
1
1
1
0. Muragan R, Ramakrishna S (2005) Comput Sci Techol 65:2385
1. Kebby LC, Hall KW (1973) J Catal 29:144
2. Yasukawa A, Yokoyama T, Kandori K, Iswkawa T (2004) Col-
loids Surf A 238:133
32. Yoon JW, Ryu JH, Shim KB (2006) Mater Sci Eng B 127:154
33. Eslami H, Solati-Hashjin M, Tahriri M, Bakhshi F (2010) Mater
Sci Poland 28:5
1
1
1
1
1
3. Boucetta C, Casimi M, Ensuque A, Piquemal J-Y, Bozon-Ver-
duraz F, Ziyad M (2009) Appl Catal A 356:201
4. Sugiyama S, Osaka T, Hirata Y, Sotowa K (2005) Appl Catal A
34. Brunauer SJ, Deming LS, Deming W, Teller E (1940) J Am
Chem Soc 62:1723
35. de Boer JH, Everett EP, Stone FS (eds). (1958) The structure and
properties of porous materials. Butterworths, London, p 68
36. El-Nabarawy TH, Fagal GA, El-Shobaky GA (1989) Bull NRC
Egypt 14:1
37. Said AA, Abd El-Wahab MMM, Alian AM (2007) J Chem
Technol Biotechnol 82:513
38. Soares APV, Portela MF, Kiennemann A, Hilaire L, Millet JMM
(2001) Appl Catal A 206:221
3
12:52
5. El-Kabouss K, Kasimi M, Ziyad M, Amar S, Bozon-Verduraz F
2006) J Mater Chem 16:2453
(
6. Pivtedu LD, Girona MI, Schlapbach L, Barbour P, Pilot JB,
Gasser B (1999) J Mater Sci Mater Med 10:161
7. Gross KA, Chai CS, Kanangra GSK, Bin-Nissa B, Hanley L
(1998) J Mater Sci Mater Med 9:839
1
1
8. Hwang K, Lim Y (1999) Surf Coat Technol 115:172
9. Haddow DB, James PF, Van-Noort R (1998) J Sol–Gel Sci
Technol 13:261
0. Yaung RA, Holcomb DW (1982) Calif Tissue Int 34:17
1. Slosarezyk A, Stobierska E, Paszkiewicz Z, Gawlick M (1996) J
Am Ceram Soc 79:2539
39. Davydov AA (1990) Infrared spectroscopy of adsorbed species
the surface of transition metal oxides. Wiley, Chichester
40. Hadjiivanov K, Busca G (1994) Langmuir 10:4534
41. Klissurski D, Petridis D, Abadzhieva N, Hadjiivanov K (1996)
Appl Clay Sci 10:45l
42. El-Awad AM, Hassan EA, Said AA, AbdEl-Salaam KM (1989)
Monatshefte fur Chemie 120:199
43. Damyanova S, Grange P, Delmon B (1997) J Catal 168:421
44. Massoth PE (1978) Adv Catal 27:265
45. Crosman A, Gelbard G, Poncelel G, Parvulescu VI (2004) Appl
Catal A 264:23
46. Wakamura M, Kandori K, Iskikawa T (1998) Colloids Surf A
142:107
2
2
2
2
2. Yoshimura M, Suda H, Okamoto K, Ioku K (1994) J Mater Sci
9:3399
2
3. Said AA, Abd-El-Wahab MM, Alian AM (2014) IOP Conf Mater
Sci Eng 64:012058
4. Bigi A, Boanini E, Rubini K (2004) J Solid State Chem 177:3092
5. Cullity BD (1967) Elements of X-ray diffraction, 3rd edn.
Addison-Wesley, Reading
2
2
2
6. Said AA, Abd El-Wahab MM, Abd El-Aal M (2014) J Mol Catal
A 394:40
47. Kato A, Shi SO, Shishido T, Yamazaki M, Iida S (2005) J Phys
Chem Solids 66:2079
123