Direct dehydrogenation of methanol to anhydrous formaldehyde over Ag2O/γ-Al2O3 nanocatalysts at relatively low temperature

Article abstract of DOI:10.1007/s11164-016-2820-4

A series of Ag₂O/γ-Al₂O₃ nanocatalysts for direct dehydrogenation of methanol with various extents of Ag₂O loading (0.5–15?wt%) were prepared by the wet impregnation method. The catalysts were characterized by XRD, TEM and N₂ sorption. The catalysts were tested for direct dehydrogenation of methanol in a fixed-bed reactor at a relatively low temperature, 330?°C, using N₂ as a carrier gas. The obtained results revealed that pure γ-Al₂O₃ exhibits quite poor selectivity toward formaldehyde (FA) formation (≈28%). On the other hand, the addition of Ag₂O improves the catalytic activity of γ-Al₂O₃ where the catalyst with 10?wt% Ag₂O exhibits 100% conversion of methanol with 100% yield of FA. Moreover, the catalyst with 1?wt% Ag₂O achieved similar results at the reaction temperature of 370?°C.

Full text of DOI:10.1007/s11164-016-2820-4

Res Chem Intermed
DOI 10.1007/s11164-016-2820-4
Direct dehydrogenation of methanol to anhydrous
formaldehyde over Ag2O/c-Al2O3 nanocatalysts
at relatively low temperature
Abd El-Aziz A. Said1 Mohamed Abd El-Aal1
•
Received: 16 August 2016 / Accepted: 25 November 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract A series of Ag2O/c-Al2O3 nanocatalysts for direct dehydrogenation of
methanol with various extents of Ag2O loading (0.5–15 wt%) were prepared by the
wet impregnation method. The catalysts were characterized by XRD, TEM and N2
sorption. The catalysts were tested for direct dehydrogenation of methanol in a
fixed-bed reactor at a relatively low temperature, 330 °C, using N2 as a carrier gas.
The obtained results revealed that pure c-Al2O3 exhibits quite poor selectivity
toward formaldehyde (FA) formation (&28%). On the other hand, the addition of
Ag2O improves the catalytic activity of c-Al2O3 where the catalyst with 10 wt%
Ag2O exhibits 100% conversion of methanol with 100% yield of FA. Moreover, the
catalyst with 1 wt% Ag2O achieved similar results at the reaction temperature of
370 °C.
Keywords Ag2O/c-Al2O3 Á Methanol Á Dehydrogenation Á Anhydrous
formaldehyde
Introduction
Formaldehyde (FA) has received wide attention due to its important applications in
many fields, such as the production of raw material for other chemical products,
including the production of new plastics, phenol or urea formaldehyde resin. In
addition, it can be used in synthetic polymer materials, new drug intermediates,
advanced perfumes and dyes [1]. Generally, anhydrous FA in industry is obtained
by the dehydration reaction of an aqueous solution of FA, which is mainly produced
from the partial oxidation of methanol by air in the presence of a silver catalyst or a
Fe-Mo catalyst [2, 3]. However, FA forms an azeotrope with water, making the
&
Abd El-Aziz A. Said
aasaid55@yahoo.com
1
Chemistry Department, Faculty of Science, Assiut University, Assiut 71516, Egypt
123

A. E. -A. Said, M. A. El-Aal
removal of water and methanol from the aqueous solution of FA ivery expensive.
Thus, the direct dehydrogenation of methanol to anhydrous FA is a desirable route
because it is able to avoid the disadvantages of oxidation and separation problems
(FA and H2 can be easily separated).
Methanol can be converted to anhydrous FA and hydrogen without any catalyst
at a very high temperature, 900 °C, with the FA yield being less than 20%, and with
only 43% selectivity toward FA [4]. Yunan [5] studied the direct dehydrogenation
of methanol to anhydrous FA over a Li0.5Na0.5AlO2 catalyst. He found that the
maximum yield of FA was 70% at the reaction temperature of 900 °C.
Consequently, the challenge is to manage the direct dehydrogenation of methanol
to anhydrous FA at a lower temperature over highly active and selective catalysts.
Ren et al. [6] reported the results of the direct dehydrogenation of methanol to
anhydrous FA over polycrystalline silver, Ag–SiO2–MgO [7], Ag–SiO2–Al2O3 [8],
Ag–SiO2–Al2O3–ZnO [9], and Ag–SiO2–MgO–Al2O3 [3] catalysts. They observed
that the polycrystalline silver after pre-treatment with oxygen at high temperature
showed excellent activity and selectivity at 600 °C. It exhibited 100% of both
methanol conversion and selectivity toward anhydrous FA. They also found that the
Ag–SiO2–MgO catalyst exhibited activity of 96% conversion and 75% yield of FA
under the optimum conditions of 20 wt% loading of silver and 650 °C reaction
temperature. On the other hand, the Ag–SiO2–Al2O3 catalyst showed a catalytic
performance of 95% methanol conversion and 81.2% of FA yield using 18 wt%
loading of silver and 650 °C. In addition, the Ag–SiO2–Al2O3–ZnO catalyst showed
a catalytic activity of 99% conversion and 87% selectivity under optimum
conditions (20 wt% loading of silver, zinc content 15% and 650 °C). Moreover,
they observed that the Ag–SiO2–MgO–Al2O3 catalyst with mass ratios of
20:55.2:8.3:16.5 for Ag, SiO2, MgO and Al2O3, respectively, showed a highly
efficient catalytic performance with 100% conversion and 100% selectivity toward
FA formation. Unfortunately, the operating temperature (650 °C) and the percent-
age of Ag (20 wt%) are both very high. According to the previous survey, the
development of a new catalytic system able to convert methanol to anhydrous FA at
a relatively low temperature using low-cost materials is in great demand. However,
the direct dehydrogenation of methanol over a Ag2O/c-Al2O3 catalyst to anhydrous
FA has not, to the best of our knowledge, been reported yet. Hence, our present
work is to report a series of Ag2O/c-l2O3 catalysts prepared by the impregnation
method that show significantly high catalytic performance in the direct dehydro-
genation of methanol to anhydrous FA. X-ray diffraction, BET and transmission
electron microscopy (TEM) have been used for their characterization.
Experimental
Catalyst preparation
Aluminum nitrate nonahydrate Al(NO3)3Á9H2O (Oxford Assay 98.5%), silver
nitrate AgNO3 (Sigma Aldrich), ammonia solution (NH4OH 23%) and methyl
alcohol were obtained as pure reagents and used without further purification. The
123

Direct dehydrogenation of methanol to anhydrous…
preparation of c-Al2O3 has been described in our previous work [10]. For the
aluminum nitrate precursor, after precipitation by ammonia solution at pH = 7, the
resulting precipitate was filtered and washed by bi-distilled water several times. The
precipitate was dried at 100 °C for 24 h and calcined at 500 °C in a static air
atmosphere for 3 h. The Ag2O/c-Al2O3 catalysts with different loadings of Ag2O
(0.5–15 wt%) were prepared by a wet impregnation method and designed as
0.5–15% SA. The typical synthesis procedure was illustrated by 10% SA. An
amount of 0.2932 g of silver nitrate was dissolved in small amounts of bi-distilled
water, and then 1.8 g of the prepared c-Al2O3 powder was added and the mixture
was admixed carefully to obtain a homogeneous paste. The mixture produced was
dried at 100 °C for 24 h before being calcinated at 500 °C in a static air atmosphere
for 3 h.
Catalyst characterization
Powder X-ray diffraction (XRD) experiments of the catalysts were performed using
˚
a Philips (The Netherlands) diffractometer (Model PW 2103, k = 1.5418 A, 35 kV
and 20 mA) with a source of CuKa radiation (Ni filtered). Particle size was
estimated using Scherrer’s formula [11].
TEM with a high-resolution experimental microscope (Tecnai G2-model Spirit
Twin-120 KV) was used to investigate the size and the morphology of the catalysts.
The catalyst powder was dispersed in ethanol using ultrasonic radiation for 20 min
and a drop of that suspension was placed onto the carbon-coated grids. The degree
of magnification of the TEM images was the same for all the investigated catalysts.
Average particle size and particle size distribution were obtained manually by
measuring the diameter of the possible number of particles from the TEM image.
The mean particle diameter (dM) was calculated as in the previous report [12] by the
formula:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
ð
ni  di2Þ
P
dM
¼
ni
where di is the particle size and ni is the number of particles in the size range
between di ? D and di - D.
Brunauer–Emmett–Teller (BET) analysis was performed using a Nova 3200
(Quantachrom Instruments, USA). The surface areas and pore volumes were
measured by N2 adsorption and the desorption isotherm was -196 °C.
Catalytic evaluation test
The catalytic dehydrogenation of methyl alcohol was performed to evaluate the
efficiency of the prepared materials as catalysts. The catalytic run was carried out at
330 °C in a conventional fixed-bed flow-type reactor at atmospheric pressure using
nitrogen as a carrier gas. The measurement procedure was similar to that reported
previously [13]. Each catalytic run was conducted using 500 mg of the powdered
catalyst and the total flow rate was fixed at 50 ml min-1 and 4% methanol in the gas
123

A. E. -A. Said, M. A. El-Aal
feed. The products were analyzed by on-line gas chromatography using a 2-m DNP
glass column. Measurements of the conversion and yield (%) were recorded after
1 h from the initial introduction of the reactant into the reactor to ensure the
attainment of the reaction equilibrium (steady-state conditions). Methanol conver-
sion (C), FA selectivity (S) and yield of FA (Y) were used to assess the activity of
the catalysts [14].
½MethanolŠin À ½MethanolŠ
out
Cð%Þ ¼
 100
½MethanolŠ
½FAŠ
in
S ð%Þ ¼
 100
½FAŠ þ ½by-productsŠ
ðC Â SÞ
Y ð%Þ ¼
100
In these equations, [Methanol]in and [Methanol]out denote the peak areas (concen-
trations) of methanol at the reactor inlet and outlet, respectively.
Results and discussion
Catalyst characterization
The XRD patterns of c-Al2O3, 1, 10 and 15% SA (fresh and used) catalysts are
shown in Fig. 1. For 1, 10 and 15% SA fresh samples, the presence of typical c
phase Al2O3 diffraction peaks at 2h = 46° and 66.5° were observed [15]. This result
indicates that the addition of silver oxide to c-Al2O3 did not affect the polymorphs
of the alumina support [16], and an increase in the Ag2O content was found to be
accompanied by a decrease in the intensities of the XRD peaks associated with the
c-Al2O3 phase. In addition, diffraction peaks of Ag2O at 2h = 32.4°, 37.6°, 54.2°
and 64.6° (JCPDS file 04-005-4923), and low intensity diffraction peaks
corresponding to metallic silver at 2h = 44.1° and 77° (JCPDS file 04-002-1347)
were also detected. It is known that silver can be easily reduced to a metallic state at
temperatures higher than 300 °C [17]. Thus, 10 and 15% SA fresh samples showed
low-intensity diffraction peaks corresponding to metallic Ag. After the direct
dehydrogenation reaction of methanol, there was a new sharp peak appeared at
2h = 37.9°, which can be ascribed to the crystalline Ag. Also, an observable
increase in the intensities of the diffraction peaks at 2h = 44.1° and 77°. This
behavior can be explained as the silver ion (Ag2?) in the fresh samples can be
reduced to the metallic state Ag by H2 released during the dehydrogenation reaction.
It can also observe that there are still diffraction peaks corresponding to Ag2O in the
used samples. The absence of the diffraction peaks due to Ag-species in 1% SA
fresh sample may be due to the small amounts of silver oxide added were below the
detection limits of the X-ray diffractometer used [18]. The diffraction peaks at
2h = 46° and 66.5° for c-Al2O3 phase, at 2h = 32.4°, 37.6°, 54.2°, and 64.6° for
the Ag2O phase and at 2h = 37.9°, 44.1°, and 77° for the Ag phase were selected
123

Direct dehydrogenation of methanol to anhydrous…
♦
• γ-Al2O3
♥ Ag2O
♦ Ag
♦
♥
♦
♦
•
15% Used
♥
♥
♦
•
•
♦
•
•
♥
15% Fresh
10% Used
♦
♦
•
♥
•
•
•
10% Fresh
•
•
•
1% Fresh
•
•
γ-Al2O3
10
20
30
80
40
2θ
50
60
70
Fig. 1 X-ray diffractograms of pure c-Al2O3, 1, 10 and 15% SA catalysts (fresh and used)
for calculating the crystallites sizes. The averages of these calculations were
reported as the crystallite sizes for each phase are cited in Table 1. It was observed
that the crystallite size of c-Al2O3 did not change with the addition of Ag2O after or
before the reaction. Moreover, the crystallite sizes for Ag2O and Ag phases slightly
increase with increasing silver oxide loading.
Table 1 The crystallite (D) and mean particle (dM) sizes for c-Al2O3, 1, 10 and 15% SA (fresh and used)
samples calcined at 500 °C
Sample
DAl2O3 (nm)
DAg2O (nm)
DAg (nm)
dM (nm)
c-Al2O3
8
–
–
–
1% SA (fresh)
10% SA (fresh)
10% SA (used)
15% SA (fresh)
15% SA (used)
11
12
12
–
–
–
6.3 3.1
9.2 3.1
–
22
–
12
18
29
30
19
35
9.2 2.9
12 4.5
–
123

A. E. -A. Said, M. A. El-Aal
TEM was used to determine the morphology of the catalysts before and after the
methanol dehydrogenation reaction and the silver species particle size distribution.
TEM images of c-Al2O3, 10% SA and 15% SA (fresh and used) catalysts as well as
the Ag2O and Ag0 nanoparticles size distribution are presented in Fig. 2. These
pictures show that c-Al2O3 support appears as small aggregates with average
particle size of about 8–14 nm. On the other hand, the catalysts (1, 10 and 15% SA)
presented the spherical Ag2O particles, well distributed on the support.
Fig. 2 TEM Images of a c-Al2O3, b 1% SA fresh, c 10% SA fresh, d 15% SA fresh and e 15% SA used
and the particle size distribution
123

Direct dehydrogenation of methanol to anhydrous…
40
35
30
25
20
15
10
5
1% SA Fresh
Total particles count =60
M= 6.3 3.1
Minimum =
Maximum = 15
10% SA Fresh
d
Total particles count =60
dM= 9.2 3.1
3
30
20
10
0
Minimum =
2
Maximum = 16
0
2
5
7
9
10
14
16
3
4
5
7
9
11 13 15
Particle size (nm)
Particle size (nm)
15% SA Used
Total particles count =60
Mean = 12 4.5
15% SA Fresh
20
40
30
20
10
0
Minimum =
Maximum = 23
5
Total particles count =90
dM = 9.2 2.9
15
10
5
Minimum =
2
Maximum = 16
0
2
5
7
9
10
14
16
5
7
9
10 14 16 18 23
Particle size (nm)
Particle size (nm)
Fig. 2 continued
Measurements of the size distribution based on the images showed a mean particle
size of 6.3 3.1 for 1% SA fresh sample with a narrow distribution range of
3–15 nm. On the other hand, the mean particle size and the distribution ranges of
the 10 and 15% SA fresh samples are 9.2 nm and 2–16 nm, respectively. As can be
seen, some small Ag2O particles aggregated into larger ones and the others reduced
to metallic Ag after the reaction test [19] (Fig. 2d); the largest particle was about
23 nm in diameter. The mean particle sizes of the samples are cited in Table 1. The
results indicate that increasing the Ag2O content slightly increases the mean particle
size. On the other hand, the mean particle size increases after the reaction. It is
worth mentioning here that the crystallite size estimated from the XRD peak width
is larger than the particle size measured by TEM [20, 21]. This behavior can be
explained on the basis of the samples with nanoparticles polydispersed in size, the
Scherrer equation applied to XRD data gives an indication of the size of only the
largest present particles [22].
The nitrogen adsorption–desorption isotherms (a) and the corresponding BJH
pore size distribution (b) of c-Al2O3, 1, 10 and 15% SA catalysts are represented in
123

A. E. -A. Said, M. A. El-Aal
Fig. 3. All isotherms are of type IV, which is a characteristic feature of mesoporous
materials with H2 hysteresis loops. The pore size distribution calculated from
desorption branch is shown in Fig. 3b. This shows that the pore diameters were
&4.9 nm and slightly changed with the surface modifications with Ag2O loadings.
The observed changes of specific surface area (SBET) and pore volume as well as
(a)
200
γ-Al2O
3
1% SA
10% SA
15% SA
150
100
50
0
0.0
0.2
0.4
0.6
0.8
1.0
P/Po
(b)
10
5
15% SA
0
10
5
10% SA
1% SA
0
15
10
5
0
10
5
γ
-Al2O3
0
0
5
10
15
20
Mean pore radius, r-p (nm)
Fig. 3 a Nitrogen adsorption–desorption isotherms of pure c-Al2O3, 1, 10 and 15% SA catalysts. b Pore
size distribution of pure c-Al2O3, 1, 10 and 15% SA catalysts
123

Direct dehydrogenation of methanol to anhydrous…
pore size are shown in Table 2. The surface area and pore volume decrease as a
result of the incorporation of Ag2O nanoparticles into the mesoporous channels of
the support [23].
Catalytic dehydrogenation of methanol to anhydrous FA
The effect of loading c-Al2O3 with different percentages of Ag2O (0.5–15 wt%) on
the direct dehydrogenation of methanol to anhydrous FA was investigated at 330 °C
and is illustrated in Fig. 4. Under our operating conditions, pure c-Al2O3 by itself
exhibits high conversion of methanol &90%, with 72 and 28% selectivity toward
DME and FA formation, respectively. The additions of Ag2O to c-Al2O3 lead to an
observable increase in the conversion and selectivity toward FA. In addition, the
10% SA sample shows the maximum activity of 100% conversion and 100% yield
of FA. As compared with our results and those previously published in the literature,
from an economical point of view, the 10% SA catalyst is the best one because it
gives outstanding activity using a low temperature and low percentage of Ag2O.
The most important parameters affecting the direct dehydrogenation of methanol
to anhydrous FA are the reaction temperature, % methanol, reaction time and
catalyst calcination temperature. We choose the 1% SA catalyst for studying
different parameters because we have tried to reduce the percentage loading of
Ag2O. This, the effect of reaction temperature on the direct dehydrogenation of
methanol over the 1% SA catalyst was studied andis shown in Fig. 5. This
figure demonstrates that the reaction starts at 125 °C with 3% conversion and 100%
selectivity toward DME. On increasing the reaction temperature up to 250 °C, it
shows a promising activity of &92.4% of methanol conversion with a remarkable
yield of DME. Thus, the 1% SA catalyst can be used as an efficient catalyst for
methanol dehydration to DME at that temperature. On the other hand, with
increasing the reaction temperature above 250 °C, the selectivity toward DME
decreases whereas the selectivity toward FA increases. At 370 °C, the 1% SA
catalyst exhibits the best activity with 100% conversion and 100% selectivity
Table 2 Variation of specific surface area and pore characteristics of Ag2O/c-Al2O3 catalysts calcined at
500 °C
Wt% Ag2O
SBET (m2 g-1
)
St (m2 g-1
)
CBET
Total pore volume
(cc g-1
Average pore
diameter (nm)
)
c-Al2O3
222
221
211
204
203
201
190
176
222
221
211
204
203
201
190
176
130
143
143
152
162
140
153
153
0.284
0.277
0.275
0.270
0.267
0.265
0.252
0.235
5.1
5.5
5.5
5.3
5.4
5.4
5.3
5.3
0.5
1
3
5
7
10
15
123

A. E. -A. Said, M. A. El-Aal
100
80
60
40
20
0
Conversion
S. DME
S. FA
Y. DME
Y. FA
γ-Al2O3
1
3
5
7
10
15
0.5
Wt. % Ag2O
Fig. 4 Catalytic dehydrogenation of methanol over pure c-Al2O3 and 0.5–15% SA catalysts at 330 °C
100
80
Conversion
60
40
20
0
S. DME
S. FA
Y. DME
Y. FA
100
150
200
250
300
350
400
Reaction temperature (o C)
Fig. 5 Effect of reaction temperature on the dehydrogenation–dehydration of methanol over the 1% SA
catalyst calcined at 500 °C
toward FA formation. From the above results, it is clear that the 1% SA catalyst has
a dual function because at 250 °C can be used as an efficient catalyst in DME
production while it can be also be applied as an outstanding catalyst in the synthesis
of anhydrous FA at 370 °C.
The effect of % methanol on the activity of the 1% SA catalyst at 370 °C was
investigated and the results are presented in Fig. 6, showing that the higher activity
toward the FA formation (100% yield) was obtained when using 4% of methanol.
Moreover, an increase in the % of methanol led to an observable decrease in
methanol conversion and the yield of FA, and at the same time the yield of DME
increases. For example, when using 19.5% of methanol, the 1% SA catalyst exhibits
123

Direct dehydrogenation of methanol to anhydrous…
100
80
Conversion
S. FA
60
40
20
0
S. DME
Y. FA
Y. DME
2
4
6
8
10
12
14
16
18
20
% Methanol
Fig. 6 Effect of % methanol on its dehydrogenation–dehydration over 1% SA catalyst calcined at
500 °C
89% of methanol conversion, with 53.4 and 35.6% yield of FA and DME,
respectively.
The effect of calcination temperature on the catalytic activity of the 1% SA
catalyst toward methanol dehydrogenation to anhydrous FA at 370 °C was studied
on the catalyst calcined at 500, 550, 650 and 750 °C and the results are illustrated in
Fig. 7, showing that raising the calcination temperature does not affect on the
catalytic activity of the catalyst. These results reflect that the active sites responsible
for the direct methanol dehydrogenation unchanged with increasing the calcination
temperature up to 750 °C and this provides another excellent value added to the
novel behavior of our catalyst.
Conversion
S. FA
100
80
60
40
20
0
500
550
600
650
700
750
Calcination temperature (oC)
Fig. 7 Effect of catalyst calcination temperature on methanol dehydrogenation over the 1% SA catalyst
at 370 °C
123

A. E. -A. Said, M. A. El-Aal
100
80
60
40
20
0
2
4
6
8
10
12
14
16
18
20
Time (h)
Fig. 8 Effect of reaction time on the catalytic dehydrogenation of methanol over the 1% SA catalyst
calcined at 500 °C
Stability of the catalyst
It was accepted that the most important feature of the catalyst is the stability during
the reaction for a long lifetime. So, the long-term stability of 1% SA calcined at
500 °C was carried out at 370 °C reaction temperature and using 4% methanol. The
obtained results are presented in Fig. 8. It shows that the methanol conversion and
the yield of FA remained constant for 20 h, which indicated that no noticeable
deactivation of the catalyst was occurred. However, it was reported that Ag–SiO2–
Al2O3–ZnO catalyst showed a much longer lifetime (about 2 h) than the Ag–SiO2–
Al2O3 and Ag–SiO2–ZnO catalysts (only 1 h) [9]. So, in comparison with our
results with previously published data we found that our catalyst is the most
stable one in direct dehydrogenation of methanol to anhydrous FA.
Conclusions
In this work, nano-Ag2O/c-Al2O3 catalyst prepared by a simple conventional
impregnation method is demonstrated to be an outstanding efficient catalyst for the
direct dehydrogenation of methanol to anhydrous FA. Detailed structural analysis
by XRD, TEM and BET methods showed that the catalyst is mesoporous in nature
and, during the direct dehydrogenation reaction, Ag2O is reduced to metallic Ag
with a slight increase in particle size being observed. The catalytic performance of
the catalyst is mainly dependant on the content of Ag2O and the reaction
temperature. The catalyst with 10 wt% Ag2O loading exhibits excellent catalytic
activity, with selectivity to anhydrous FA reaching 100% with a methanol
conversion of 100% at relatively low temperature of 330 °C. Moreover, the catalyst
123

Direct dehydrogenation of methanol to anhydrous…
with 1 wt% Ag2O gives 100% yield of anhydrous FA at 370 °C, and no appreciable
deactivation is observed even after reacting for more than 20 h.
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