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Catalytic Activity of (Cu (1-x) Znx)3 (PO4)2 (x=0; 0.39; 0.5; 0.57) Catalysts in Isopropanol Decomposition

Authors:
Electronic copy available at: https://ssrn.com/abstract=3185286
Catalytic activity of (Cu (1-x) Znx)3 (PO4)2 (x=0;
0.39; 0.5; 0.57) catalysts in isopropanol
decomposition
A.AJARROUD1* S.TAYANE3, 4 T.JEI1* M.BENSITEL2
1Laboratory of Materials and applied Physic, Hassan II
University of Casablanca, Faculty of Sciences Ben
M'sik, Casablanca, Morocco.
2Laboratory of Catalysis and Corrosion of Materials,
University of Chouaïb Doukkali, Faculty of sciences of
El Jadida, Morocco
*Corresponding authors, E-mail address:
asmaaajarroud@gmail.com; jeitaib@yahoo.fr
3Laboratory of Biomolecules and Organic Synthesis,
Hassan II University of Casablanca, Faculty of
Sciences Ben M'sik, Casablanca, Morocco.
4National School of Arts and Crafts of Casablanca
(ENSAM), Casablanca, Morocco
Abstract New substitutional solid solutions with the
general formula (Cu (1-x)Znx)3 (PO4)2 (x=0; 0.39; 0.5; 0.57) have
been synthesized by co-precipitation.The transformation from
initially amorphous particles to crystalline structure takes
place; the results depend on the metal ratio used and the
temperature of calcination at the same pH value. The resulting
products were characterized by X-Ray diffraction, transmission
electron microscopy, scanning electron microscopy and
Infrared spectroscopy then tested as catalysts in decomposition
of isopropanol.
Keywords—Isopropanol dehydrogenation; catalysts; metal
phosphates; propene; acetone; co-precipitation.
I. INTRODUCTION
Divalent metal phosphates and related solid solutions
have been studied extensively in literature [1-2]. The
use of metal phosphates as catalysts has been developed
in various heterogeneous reactions, such as trans-
esterification, alkylation, hydrogenolysis [3-5]. The
importance of identifying solid phases with different
Cu/Zn ratios was stressed by Stone and Waller [6]. In
mixed systems the spherical particles were always
amorphous, while particles of other morphologies were
crystalline [7]. Different papers had demonstrated the
conversion from amorphous to crystalline solids [8],
meanwhile other works showed that mixed Cu/Zn forms
ribbon like particles [9-10]. Recent works showed that
the formation of particles at different initial ratios of
Cu/Zn exhibits dramatic changes. These conversions are
the consequences of many factors, among them we can
site: the ratio of the two metals nitrates in the initial
reactant solutions and the temperature of calcination [8-
9]. Acidic solids derived from metal orthophosphates
are widely used in the catalytic process of various
reactions occurring at the gas/solid interface. Acidbase
and redox characters play an important role in the
chemical properties of the orthophosphate catalyst
surface [11].
Alcohol dehydrogenation has been discussed as test
reaction in monometallic single crystals and bimetallic
catalysts. Cu metal is highly selective for alcohol
dehydrogenation but suffers from low activity thus; the
addition of a second metal may not only increase
dehydrogenation activity and catalyst lifetime but also
maintain high selectivity [12]. In alcohol formation, a
synergistic effect between Cu and Zn for methanol
synthesis has been documented by a number of
researchers [13-14], and it is known that the rate of
alcohol dehydrogenation is metal specific [15-16].
Alcohol decomposition is also used as test reaction for
the evaluation of the acidbase properties of catalysts
[17-18]. Alcohol dehydrogenation products (aldehydes
and ketones) are preferentially formed on basic
catalysts, while dehydration products (olefins and
ethers) are favoured when acidic sites are present [19].
It is generally accepted that isopropanol decomposition
over basic sites proceeds through an elimination
reaction yielding acetone [20-21]. Meanwhile, over acid
sites, isopropanol dehydrates to propylene and to di-
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Fig. 1. X ray diffracti on of (Cu1-XZnx)3(PO4)2; (A) x=0.57; (B) x=0.5;
(C) x= 0.39; (D) x=0 at 400°C
isopropyl ether [22-23].
In this study, we have prepared (Cu (1-x)Znx)3 (PO4)2 solid
phases with (x=0; 0.39; 0.5; 0.57), characterized them
by various techniques, and studied them as catalysts, for
the catalytic decomposition of isopropanol.
In this work we investigated two metal salts, Zinc and
copper nitrates, in order to gather information on the
effect of the metal salts composition on the physico-
chemical properties, texture and acidity of the system
obtained by simultaneous co-precipitation. The obtained
solid phases showed differences, not only in
crystallinity but also in the acid properties of the
surface, and this is due to the molar composition of the
metal, temperature treatment and the precipitating agent,
II. MATERIALS AND METHODS
A. Preparation of solids
(Cu1-XZnx)3(PO4)2 with different molar compositions of
Zinc-Copper (x=0; 0.39; 0.5; 0.57). These materials
were prepared by co-precipitation method using
ammonia as a precipitating agent. Aqueous solutions
containing copper nitrate, zinc nitrate and
orthophosphoric acid in the mole ratio desired, they
were stirred in a beaker with a magnetic stirrer. Then,
ammonia solution was added dropwise with stirring
until the pH of the precipitate became 6. The
precipitates thus formed were filtered, washed with
distilled water, and dried at 180°C for 24 h [24]. Then
the solids were calcined in air oven at 400°C and 700°C
for 4h.
B. Characterization of solids
The X-ray diffraction data were collected at room
temperature with a Philips X’Pert Pro diffractometer
(CuKα) equipped with a diffracted beam
monochromator. The data were collected in the 0-90°
range in steps of 0.0 (2θ), with a constant counting
time of 40 s per step. Infrared spectra were recorded on
a Nicolet 510 FTIR spectrophotometer on 1% samples
in KBr pressed disks. Transmission electron
microscopic (TEM), images were obtained on a JEOL-
2010 microscope with an accelerating voltage of 200
kV. Scanning Electron Microscopy (SEM; Tecnai G2
12 TWIN).
C. Dehydration of Isopropanol
The activities of the catalysts were measured by the
decomposition of isopropanol, at atmospheric pressure,
the catalyst (50 mg, 200500 µm particle size) placed
in a reactor (Pyrex tube, 10 mm I.D). Before carrying
out such catalytic activity measurements, the catalyst
was placed inside a vertical furnace with controlled
temperature, and was activated by heating in a stream
of nitrogen for 2 h at 400°C, then cooled to the catalytic
reaction temperature. The reactant, propanol-2 (3.7
kPa), was diluted in nitrogen by bubbling the gas
through the liquid reactant in saturator maintained at
12°C, with a contact time of 5.76 h. The isopropanol
decomposition was performed at a temperature between
220 and 340°C. The products were analyzed by gas
chromatography (Varian-3700 GC with a flame
ionization detector FID) using a 10% 0V–101 on
Chromosorb-WHP (80/100 mesh) packed column (4m)
maintained at 80°C. It is worth noting that a blank test
showed insignificant thermal reaction developed in the
absence of the catalyst.
III. RESULTS AND DISCUSSION
A. XRD characterization:
Fig. 1, and 2 represent the powder diffraction patterns
of (Cu1-XZnx)3(PO4)2 (x=0; 0.39; 0.5; 0.57) calcined at
400°C and 700°C. The characteristic diffraction spectra
of these samples indicated that solid phases with ratios
(x=0.50, 0.57) are amorphous as shown in Fig. 1. When
the ratio of copper increased (x=0, 0.39) at 400°C, the
morphology transformed from amorphous to crystalline
[12], (Anorthic structure, sp. gr. P-1) PDF Index Name:
Copper Phosphate. ref cod: 01-080-0991. Concerning
other phases, Fig. 2, presents a good crystallinity at
700°C Thus, it is clear that the presence of Zn in the
mixture make a delay of the Cu crystallization process.
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B. Infrared spectroscopy studies
Table I. IR bands assignments of (Cu1-XZnx)3(PO4)2 ;
(x=0, 0.39, 0.5, 0.57) catalysts
Mode x=0
x=0 x=0.39
x=0.50
x=0.57
400°C
!!
!
!
!!
ν2(PO4)
-
-
527
528
ν4(PO4)
611
611
605
600
ν1(PO4)
-
-
671
673
νas(PO4)
1085
-
1064
1055
δ(H2O)
3485
3481
3464
3467
1658
1653
1652
1652
ν3(PO4)
-
-
1128
1145
700°C
!
!
!
!!
ν2(PO4)
-
542
540
540
ν4(PO4)
568
584
568
582
609
630
601
632
ν1(PO4)
667
666
669
669
ν1(PO4)
-
967
970
968
ν3(PO4)
1014
1022
1062
1056
υas(PO4)
-
1240
1235
1236
δ(H2O)
-
-
1076
1079
3482 3482
3481
3481
The infrared spectra of catalysts (Cu (1-x)Znx)3 (PO4)2
treated at 400°C and 700 °C, are shown in Fig. 3, and 4.
Table I. summarizes the normal modes of
orthophosphates and their attributions according to ref.
[20] and [25]. The broad absorptions observed in the
1076 and 3482 cm-1 are both assigned to δ(H2O).
Meanwhile, the vibration stretching mode (asymmetric
stretching) ν3(PO4) of phosphate anions was observed
around 1022 cm-1. The IR spectrum of Cu, Zn-P
calcined at 400°C, shows bands around 1060 cm-1 and
bands around 1128 cm-1 contributed to (symmetric
stretching). The υ1 vibration (symmetric stretching)
occurred at 671cm-1, while υ4 (symmetric bending)
located around 611 cm-1. It can be seen from this figure
that the intensities of (PO4
3-) absorption bands around
671, 1060 and 1128 cm-1 decreased when the amount of
Zinc in the samples increased.
At 700 °C the spectrum of solids Cu, Zn-P, still
exhibits the same bands for orthophosphates (PO4
3-)
ions previously observed. Besides, bands appeared
clearly around 568 cm-1 and around 970 cm-1 attributed
respectively to υ4 asymmetric bending vibration and ν1
symmetric stretching vibration.
C. Scanning electron microscopy studies
Figs. 5 and 6, represent Scanning electron micrography,
which illustrates two major effects; the change of
particles sizes and the new phase formation. This
conversion depends on the ratio of the two metal Cu/Zn
in different solid phases and temperature of calcination.
Fig. 5, shows larger grain size of (Cu(1-x)Znx)3(PO4)2;
Fig. 3. IR spectra of (Cu(1-x)Znx)3(PO4)2 (x=0; 0.39; 0.5; 0.57) treated at 400°C
Fig. 4. IR spectra of (Cu(1-x)Znx)3(PO4)2 (x=0; 0.39; 0.5; 0.57) treated at 700°C.
Fig. 2. X ray diffraction of (Cu1-XZnx)3(PO4)2; (A) x=0.57; (B) x=0.5;
(C) x= 0.39; (D) x=0 at 700°C
D
C
B
A
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(x=0.5; 0.57) treated at 400°C. This morphology is
further corroborated by XRD analysis,
which has shown that these particles are amorphous.
The increase of Zinc (x=0, x=0.39) cause dramatic
changes, of the morphology and sizes of particles,
which lead to the formation of irregular nanosphere.
Furthermore, Fig. 6, shows the ratios (x=0.5; 0.57),
calcined at 700 °C. These two solid phases had
crystalline layers, which illustrated clearly the
orientation of the grains. The conversion from
amorphous to crystalline solids seems to proceed
through the formation of uniformly dispersed
composite solids with ratios (x=0; 0.39). The average
sizes of the obtained particles decreased with increasing
concentration of the added copper ion solution [27].
The explanation why the final size of the particles did
not change with increased copper concentration can be
understood from precipitation behaviour of each ion. It
is known that copper ion solution has a lower pH value
than zinc ion solution and that also copper ions
precipitate at lower pH value than zinc ions [27-28].
D. Transmission electron microscopy:
Fig.7, Shows the TEM pattern of the samples at
temperatures 400°C, The Cu ratio affects the morphology
of the catalysts, when it increased the morphology
transformed from nanorods (x=0.50; 0.57) to
nanoparticles (x=0; 0.39). However, at 700°C, ratios
(x=0; 0.39; 0.50 ; 0.57) exhibits similar nanoparticles as
shown in Fig.8. Hence, The temperature of calcination
and Cu ratios are the main parameters to control the
transition from nanorod to nanoparticle morphologies of
(Cu(1-x)Znx)3(PO4)2 catalysts. This fact is in agreement
with previous DRX analysis, in which we have shown
structural changes. In the next section, catalytic tests on
the products obtained at 400 °C.
Fig. 7. TEM images of (Cu(1-x)Znx)3(PO4)2; (x=0; 0.39; 0.5; 0.57) calcined at 400° C
X=0
X=0.39
X=0.57
X=0.50
Fig. 8. TEM imag es of (Cu(1-x) Znx)3(PO4)2 (x=0; 0.39; 0.5; 0.57) calcined at 700° C
X=0
X=0.39
X=0.50
X=0.57
Fig. 5. SEM ima ges of (Cu(1-x)Znx)3(PO4)2; (x=0; 0.39; 0.5; 0.57) calcined at 400°C
X=0.50
X=0.39
X=0.57
Fig. 6. SEM ima ges of (Cu(1-x)Znx)3(PO4)2.; (x=0; 0.39; 0.5; 0.57) calcined at 700°C
X=0
X=0.57
X=0.5
0
X=0.39
X=0
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IV. Catalytic test
In order to test the catalytic abilities of the four
products obtained at 400 °C, we proceeded by studying
the decomposition of isopropanol and the results were
reported in Fig. 9. The four plotted curves represent the
conversion of isopropanol on different phosphate based
catalysts (Cu(1-x)Znx)3(PO4)2 (x=0; 0.39; 0.50; 0.57). In
more details, for ratios (x=0; 0.39), corresponding to
crystalline structure, catalytic activity is higher than the
one observed in the amorphous catalysts at ratios
(x=0.5; 0.57). This behaviour is conserved for all
temperature values, ranging from 220 to 340 °C. At
300 °C, conversion value obtained at (x = 0) i.e. for
Cu3(PO4)2 catalyst is equal to 90.57 %, this value is the
highest [12], and it drops to 74.22 % when the ratio is
equal to 0.39. For the same temperature, the lowest
conversion value is observed at (x=0.57) where it drops
to 48.1 %. Moreover, different studies showed that the
high activity of different catalyst in the isopropanol
decomposition is due to their high acidity [12-14],
rather than their porous structure [15,16]. Also
phosphoric acid has proven to yield acid catalysts in a
single step with a high activity in the alcohol
decomposition [16].
Figs. 10 and 11 represent the evolution of isopropanol
conversions and selectivities as a function of the
reaction temperature (Cu(1-x)Znx)3(PO4)2 for (x=0; 0.39).
In the case of (x=0), the selectivity of isopropanol to
acetone which is a dehydrogenation product is equal to
44.17 % and to propylene which is a dehydration
product is equal to 55.83 % at 220 °C. Whereas, in the
case of (x=0.39) isopropanol is selective to acetone
with 87.73 % at the same temperature. In both cases
these selectivities increase with respect to the increase
of the temperature.
Fig.10. Isopropanol conversion (X), selectivity to acetone (Sa) and propylene
(SP) on (Cu(1-x)Znx)3(PO4)2; (x= 0) calcined at 400°C
Fig.11. Isopropanol conversion (X), selectivity to acetone (Sa) and propylene
(SP) on (Cu(1-x)Znx)3(PO4)2; (x= 0.39) calcined at 400°C
Figs. 12, and 13, represent the evolution of isopropanol
conversions and selectivity with temperature on (Cu(1-
x)Znx)3(PO4)2 for (x=0.50; 0.57). When the temperature
of reaction is less than 260 °C, the selectivity of
acetone increases, through the dehydrogenation of
isopropanol. This reaction takes place on the basic sites
of these two catalysts [22-23], containing less
proportions of Cu. The relatively high production of
0
0,2
0,4
0,6
0,8
1
0
0,2
0,4
0,6
0,8
1
200# 250# 300# 350#
Selectivity
Conversion
Temperature (°C)
X!
Sp!
Sa!
0#
0,2#
0,4#
0,6#
0,8#
1#
0
0,2
0,4
0,6
0,8
1
200# 250# 300# 350#
Selectivity
Conversion
Sp!
X!
Sa!
Temperature (°C)
!
0
20
40
60
80
100
200 220 240 260 280 300 320 340 360
Conversion (%)
Temperature (°C)
x=0
x=0.39
x=0.50
x=0.57
Fig.9. Isopropanol conversion of (Cu(1-x)Znx)3(PO4)2; (x=0; 0.39; 0.5;
0.57) calcined at 400°C
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propylene appears.
IV. CONCLUSION
(Cu (1-x)Znx)3(PO4)2; (x=0; 0.39; 0.5; 0.57) catalysts
were successfully prepared by precipitation method,
they showed a structural and morphological
transformations from initially amorphous particles to
crystalline structure, which is due to the variation of
the substitution ratio and the temperature of
calcination at the same pH value. Decomposition tests
of isopropanol of these catalysts revealed that for (Cu
(1-x)Znx)3(PO4)2; (x=0; 0.39) Cu was more active than
Zn and that all catalysts were 100% selective and
active towards propene at starting reaction temperature
of 260°C, thus indicating the prevalence of acid sites,
the morphology and the acidity of surface structure.
On the other hand, (Cu(1-x)Znx)3(PO4)2; (x=0.5; 0.57)
have strong acetone selectivity at low reaction
temperature, thus indicating the prevalence of basic
sites. Finally, it is worth noting that, the formation of
acetone, which is a product of dehydrogenation
reaction, is related to the presence of strong basic
sites. When the reaction temperature increase. The
activity of these catalysts is due to the presence of
surface acid sites [20-21].
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0
0,2
0,4
0,6
0,8
1
0
0,2
0,4
0,6
0,8
1
200 220 240 260 280 300 320 340 360
Selectivity
Converision
Temperature
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Fig.13.!Isopropanol!conversion!(X),!selectivity!to!acetone!(Sa)!
!and!propylene!(SP)!on!(Cu(1-x)Znx)3(PO4)2;!(x=0.57)!calcined!at!400°C!!
0
0,2
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Selectivity
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Fig.12.!Isopropanol!conversion!(X),!selectivity!to!acetone!(Sa)!and!propylene!
!(SP)!on!(Cu(1-x)Znx)3(PO4)2;!(x=0.5)!calcined!at!400°C!!
!
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A series of mixed zinc–aluminum phosphate (ZnAlP) catalysts containing 40–90 aluminum molar % were synthesized by a coprecipitation method and characterized by nitrogen adsorption–desorption, X-ray diffraction, FTIR spectroscopy, thermogravimetric analysis (TGA), differential thermal analysis (DTA), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature programmed desorption (TPD) of ammonia. The presence of aluminum greatly affected the surface properties of Zn3(PO4)2 by delaying the crystallization process of Zn3(PO4)2. All amorphous samples were shown to be mesoporous and they contained two types of aluminum surface hydroxyl groups and one type of phosphorus hydroxyl group, as shown by DRIFT spectra. The specific surface area and the acidity of ZnAlP increased on increasing the aluminum content. On the other hand, a great difference in the texture and the concentration of surface acid sites was found by changing the precipitating agent and calcination temperature. Thus these factors also play an important role in the final properties of these catalysts.© 2001 Society of Chemical Industry
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Libethenite Cu2(PO4)(OH) was simply synthesized by heterogeneous reaction using a mixture of Cu(NO3)2·6H2O, (NH4)2HPO4 and water at room temperature for 60 min. The thermogravimetric study indicates that the synthesized compound is stable below 250°C and its final decomposed product is Cu3(PO4)2. The pure monoclinic phases of the synthesized Cu2(PO4)(OH) and its final decomposed product Cu3(PO4)2 are verified by XRD technique. The presence of OH- ion and H2O molecule in the structure Cu2(PO4)(OH) and the PO43- ion into Cu3(PO4)2 structure are confirmed by FTIR technique. The decomposition test of isopropanol shows that the copper orthophosphate Cu3(PO4)2 catalysts have a strong propene selectivity, thus indicating the prevalence of acid sites. The thermal stability, the morphology based on Libethenite Cu2(PO4)(OH) and acidity surface structure Cu3(PO4)2of the studied compounds may affect their activities for potential applications in catalysis.
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