Ethanol dehydrogenation on copper catalysts with ytterbium stabilized tetragonal ZrO2 support

Article abstract of DOI:10.1134/S0036024416120074

The physicochemical and catalytic properties of Cu-containing crystalline zirconia, obtained via sol–gel synthesis in the presence of Yb³⁺ ions and polyvinylpyrrolidone, are studied. DTG/DSC, TEM, XRD and BET methods are used to analyze the crystallization, texture, phase uniformity, surface and porosity of ZrO₂ nanopowders. It is shown that increasing the copper content (1, 3, and 5 wt % from ZrO₂) raises the dehydrogenation activity in the temperature range of 100–400°C and lowers the activation energy of acetaldehyde formation. It is found that the activity of all Cu/t-ZrO₂ catalysts grows under the effects of the reaction medium, due to the migration and redispersion of copper.

Full text of DOI:10.1134/S0036024416120074

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2016, Vol. 90, No. 12, pp. 2370–2376. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © S.G. Chuklina, A.I. Pylinina, L.I. Podzorova, N.A. Mikhailina, I.I. Mikhalenko, 2016, published in Zhurnal Fizicheskoi Khimii, 2016, Vol. 90, No. 12,
pp. 1804–1810.
CHEMICAL KINETICS
AND CATALYSIS
Ethanol Dehydrogenation on Copper Catalysts
with Ytterbium Stabilized Tetragonal ZrO Support
2
a
a
b
b
a
S. G. Chuklina *, A. I. Pylinina **, L. I. Podzorova , N. A. Mikhailina , and I. I. Mikhalenko ***
a
RUDN University, Moscow, 117198 Russia
b
Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow, 119334 Russia
e-mail: *sofyaogan@gmail.com; **pylinina@list.ru; ***imikhalenko@mail.ru
Received March 15, 2016
Abstracts—The physicochemical and catalytic properties of Cu-containing crystalline zirconia, obtained via
3+
sol–gel synthesis in the presence of Yb ions and polyvinylpyrrolidone, are studied. DTG/DSC, TEM,
XRD and BET methods are used to analyze the crystallization, texture, phase uniformity, surface and poros-
ity of ZrO nanopowders. It is shown that increasing the copper content (1, 3, and 5 wt % from ZrO ) raises
2
2
the dehydrogenation activity in the temperature range of 100–400°C and lowers the activation energy of acet-
aldehyde formation. It is found that the activity of all Cu/t-ZrO catalysts grows under the effects of the reac-
tion medium, due to the migration and redispersion of copper.
2
Keywords: bioethanol, alcohol dehydrogenation, oxide catalysts, zirconia, copper-containing catalysts
DOI: 10.1134/S0036024416120074
INTRODUCTION
advantage of its use in catalysis is apparent in the inter-
action between the active phase and support (which
affects the catalytic activity and selectivity), and in its
high chemical inertness, compared to other known
supports (Al O , SiO ) [14]. The surface of zirconia is
characterized by a wide range of adsorption sites of dif-
ferent natures, and the ratio between these can
change, depending on the conditions of sample prepa-
ration and pre-treatment [15].
Due to the continuing development of the hydro-
gen industry, the possibility of using biofuels (such as
bioethanol) as stable and cost-effective sources of
hydrogen was considered in [1, 2]. The production of
bioethanol as a renewable, environmentally friendly
and widely available natural resource is growing
steadily. Catalytic conversion of an ethanol–water
mixture at different temperatures proceeds via differ-
ent mechanisms and gives a variety of products. Of
2
3
2
The aim of this work was to synthesize and study
great interest nowadays is studying methods of the cat- the activity of copper catalysts, prepared from zirconia
alytic steam reforming of ethanol to produce hydro- hydrogel containing structure-forming polymer and
gen-rich gas mixtures. One possible way of processing ytterbium to stabilize the t-ZrO phase, in the vapor
2
biofuels is the dehydrogenation of ethanol to acetalde- phase dehydrogenation of ethanol.
hyde, which is an intermediate for the production of
hydrogen with slight carbonization. This reaction is
EXPERIMENTAL
reversible and occurs at low temperatures [3–5].
Another possibility for the efficient conversion of eth-
anol is its conversion on acid catalysts to light olefins,
which are used as a precursors for the production of
Sample Preparation
Precursors of ZrO powders were prepared from
2
chemicals and polymers [6, 7]. Acetaldehyde subse- reagent grade ZrOCl and Yb(NO ) using the sol–gel
2 3 3
quently undergoes steam reforming to form mixtures of method.
A
diluted 25% NH OH solution
4
H and CO [8, 9]. The preferred paths of ethanol con- (GOST 24147-80; KhimMed, analytical grade) was
2
version are mainly observed on supported catalysts, used as the precipitating agent. To regulate the struc-
facilitating the adsorption of ethanol and its subsequent ture of the powders, polyvinylpyrrolidone (PVP)
dehydrogenation. Developing efficient and stable cata- (average molecular weight, 12000 g/mol, determined
lysts for these processes is an important task.
by viscometry) was introduced into the samples. PVP
as an aqueous solution with a concentration of 0.1%
Zirconia is currently attracting considerable inter-
was dissolved in the precipitant NH × H О. The PVP
est as a support in a variety of catalytic processes [10–
3
2
1
3]. In addition to its established properties of content was 0.05 and 0.15 wt % by weight of the ZrO
2
strength, heat resistance, and radiation stability, the final product. Next, a mixture of 1 M solutions of zir-
2370

ETHANOL DEHYDROGENATION ON COPPER CATALYSTS
2371
2
μm
2 μm
(а)
(b)
Fig. 1. Micrographs of ZrO xerogel samples (a) without and (b) with structure-forming additive 0.05% PVP.
2
conium oxychloride and ytterbium nitrate was added The process was conducted in air. The initial tempera-
reverse synthesis) under continuous stirring into the ture was room temperature, the heating rate was 10–
(
precipitant with PVP and maintaining pH 9–10. The 15 K/min, and the final temperature was 950°C.
duration of liogel synthesis was 2 h at room tempera-
ture. The liogel was dried at 180°C for 2 h and then
heat-treated at 500°C.
The phase compositions of the samples were mea-
sured on an XRD-6000 diffractometer (Shimadzu)
using CuK radiation (λ = 1.54 Å) in the range of
α
Sol–gel conditions of ZrO synthesis were consis-
2
angles 2θ = 22°–56° with a scanning step of 0.02°. The
tent with the stoichiometry of the hydrolysis-conden- phases were identified using the international stan-
sation reaction ZrOCl + 2NH OH = ZrO ↓ + dards data bank (JCPDS). Exposure was ~1 s at each
2
4
2
2
NH Cl + H O.
The resulting hydroxide gel precipitate was washed
point. The average size of the powder particles’ coher-
ent scattering regions (CSRs) was calculated accord-
ing to the formula
4
2
+
4
–
with distilled water to remove NH and Сl ions, dried
λ
with ethanol, and then heat-treated at 180°C for 2 h.
D =
,
The precipitate was then ground. The target product
β cos θ
was a powder of zirconia ZrO(OH ) (xerogel) that
2
where λ is the X-ray wavelength; β is the physical
broadening, rad; and θ is the angle of X-rays reflec-
contained ytterbium cations.
The crystal structure of zirconia forms at calcina- tion, degrees.
tion temperatures of 500°C or higher. The final calci-
nation temperature was 950°C.
Low-temperature
Brunauer–Emmett–Teller
(
BET) adsorption–desorption was used to measure
Synthesized
amorphous
ZrO
2
containing
2
the specific surfaces of the powders on a TriStar-3000
0
.05 wt % PVP and crystalline ZrO were character- unit (Micromeritics, United States). The dependences
ized by means of DTG/DSC, XRD, TEM, and BET, of the pore volume distribution on their diameter in
and used in the preparation of Cu/ZrO samples. The the mesopore region were obtained.
2
support was treated with a copper chloride solution
The distribution of the agglomerates in the synthe-
sized powders was obtained using an Analisette-22
laser analyzer. The lower detection limit was 0.01 μm.
with a concentration calculated to obtain copper con-
tents of 1, 3, and 5 wt % by weight of ZrO . For the
2
deposition of copper, the ZrO was held in the precur-
2
sor solution in (CuCl · 2H O reagent grade) for 24 h
2
2
Catalytic Experiments
at 25°C, dried at 60°C and subjected to heat treatment
at 400°C for 3 h. Samples of CuCl /ZrO were reduced
A flow setup equipped with a Crystal Chromatec
2
2
in a hydrogen stream for 1 h at 300°C directly in the 5000 gas chromatograph (carrier gas, helium; separa-
catalytic setup.
tion column, Porapak-Q phase at a temperature of
1
25°C; detector, FID) was used to test catalytic prop-
–1
erties. Alcohol vapor in a helium flow (rate, 1.2 L h )
was fed from a bubbler into a glass reactor containing
Physicochemical Characteristics of the Samples
The precipitated products were investigated via dif- catalyst powder, a thin layer of which was distributed
ferential thermal analysis on a Netzsch STA 409 on wide-pore filter to avoid diffusion limitations.
PC/PG unit equipped with an Aeolos mass spectro- Before each experiment, the sample was heated in a
scopic analyzer (Germany) and specialized software. helium flow at 400°C for 40 min. The catalytic exper-
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 12 2016

2372
CHUKLINA et al.
DSC (μV/mg)
exo
Area: 42.21 μV/mg
0.6
0.4
0.2
0
Area: 5.407 μV/mg
2
1
−0.2
−0.4
−0.6
Area: 44.28 μV/mg
Area: 13.45 μV/mg
100
200
300
400
500
600
700 T, °C
Fig. 2. Differential scanning calorimetry: ZrO precursor gel prepared (1) without and (2) with polyvinylpyrrolidone.
2
(а)
(b)
600
400
200
0
ZrO −F
2000
500
000
00
2
1
1
5
0
5
4
2
1
30
35
40
45
50
θ, deg
27
29
31
33
35
2
2θ, deg
Fig. 3. Fragments of diffractograms of ytterbia-stabilized zirconia powders obtained at a temperatures of (a) 450 and (b) 950°C:
F is (a) a solid solution with pseudocubic structure and (b) a solid solution with tetragonal structure.
iments were performed with gradual increase of the diameter of the gel was reduced from 14 to 4–5 nm, so
temperature in the range from 250 to 400°C, under the samples with PVP had micropores with diameters of
3
condition that a steady state was achieved at each tem- 5 nm (total volume, 19 m /g) that were not present in
perature.
the samples without PVP. After crystallization, the
specific surface area of the ZrO + PVP particles was
2
2
reduced to ~20 m /g.
RESULTS AND DISCUSSION
Using the results from the differential scanning cal-
orimetry (DSC) of xerogel samples of zirconium oxides
with different contents of PVP, it was found that the
crystallization of amorphous zirconium oxide starts at
Figure 1 shows photomicrographs of the xerogel
samples of zirconia prepared with and without PVP. It
can be seen that the powders have a hierarchical struc-
ture. Particles (~15%) with sizes of up to 100 μm were
found in the powder without the polymer additive,
while smaller agglomerates about 3 μm in diameter
predominated in the samples with PVP.
BET dispersion analysis showed that the specific removal of adsorbed and bound water.
surface of ZrO xerogels was S = 77 m /g without
PVP and ~200 m /g with PVP. The average particle development of zirconia crystalline structure was
4
80°C (the location of the dominant exopeak, Fig. 2),
which agrees with the literature data [16, 17]. In the
temperature range of 0 to 250°C, there is a broad endo-
thermic effect in the DSC curve that corresponds to the
2
2
sp
When the temperature was raised to 950°C, the
2
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ETHANOL DEHYDROGENATION ON COPPER CATALYSTS
a)% (b)
2373
(
50
40
30
20
10
0
10
50
90 130 170 d, μm
50 μm
2 μm
%
5
0
0
0
0
4
3
2
10
0
10
50 90 130 170 d, μm
50 μm
2 μm
Fig. 4. Micrographs of the sample and the particle size distribution of 5% Cu/ZrO (a) before and (b) after catalysis.
2
observed and the samples’ crystallinity grew. Intro- On ZrO , the dehydrogenation reaction proceeded
2
ducing different amounts of PVP did not alter the dif- with the formation of acetaldehyde. The maximum
fraction patterns the ZrO samples (Figs. 3a, 3b). The conversion of alcohol did not exceed 10%. Raising the
2
diffraction data (reflection angles, intensity and width concentration of PVP from 0.05 to 0.15% did not
at the half maximum of X-ray reflections) are given in affect the activity of the zirconia in ethanol conver-
sion. All samples were stable; the samples’ activity did
not change in the presence of the reaction medium.
Note that dehydration reaction with the formation of
Table 1.
The effect the concentration of polymer intro-
duced at the synthesis stage (in this case PVP) had on ethylene also occurred on these samples, the conver-
the catalytic activity of the ceramic (calcined) zirco- sion of ethanol by this way was less than 5%.
nium oxide in the conversion of ethanol was studied.
Figure 4 shows micrographs of 5% Cu/ZrO ,
2
obtained with a scanning electron microscope before
and after catalysis. It can be seen that the particle size
Table 1. Zirconia diffraction data
was 10 μm and, compared to xerogel ZrO , the texture
2
2
deg
θ,
2θ,
deg
of the particles changed during the preparation of the
copper-containing sample (the surface layer become
less dense). Aggregation of the catalyst particles, and
thus a reduction in specific surface area, was observed
after catalysis.
β
d, nm Phase
β
d, nm Phase
Sample 1:
ZrO with 0.05% PVP
Sample 2:
ZrO with 0.15% PVP
2
2
30.3
34.6
35.2
5
6
5
2.9
2.4
2.9
t
t
m
30.3
34.6
35.2
5
5
7.5
2.9
2.9
1.9
t
t
m
On all copper-containing catalysts, the dehydroge-
nation reaction formed only acetaldehyde. The tem-
perature dependences of the conversion of ethanol to
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 12 2016

2374
CHUKLINA et al.
W, %
60
50
40
30
20
1
1
3
3
5
5
%
%
%
%
%
%
2
1
2
Initial ZrO2
1
1
2
10
0
2
50
300
350
T, °C
Fig. 5. Temperature dependence of the conversion of ethanol to acetaldehyde at W% Cu/ZrO , with different contents of copper
2
in the (1) first and (2) repeated experiments.
acetaldehyde are shown in Fig. 5. The dehydrogena- 300°C, the conversion of ethanol to aldehyde was 30%
tion reaction begins on the initial surface at T = 250°C. on all samples. As in the experiment on the initial sur-
In the temperature range of 250–300°C, the activity of face, however, the activity of the samples with 1 and
the copper-containing sample remained unchanged 3% Cu decreased to 30% in the repeated test, starting
at 340°C. As on the initial surface, the conversion of
ethanol to acetaldehyde in the sample with 5% Cu,
grew throughout the investigated range of tempera-
tures in the repeated experiment. The catalytic proper-
ties of all of the studied samples are shown in Table 2.
and did not exceed 5%. The sample with the minimum
copper content showed the highest activity in the tem-
perature range of 300–360°C, and its activity
decreased. The maximum conversion of alcohol to
acetaldehyde was observed on the sample with the
maximum content of Cu (5%) at T > 360°C, and it
Table 2 shows that during the preparation of cop-
reached 50% at 400°C. In a repeated experiment, the per-containing catalysts, dehydrogenation centers
reaction occurred at a lower temperature: at T = similar in nature form on the initial surface (E ≈
a
9
0 kJ/mol), and the number of these centers is the
same although the samples are of different composi-
Table 2. Conversion of ethanol to acetaldehyde W, % on
tions (close values of ln N ≈ 2). Under the effect of the
0
Cu/ZrO catalysts at temperatures of 300 and 380°C: exper-
2
reaction medium (in the repeated experiments), new
centers different in nature and quantity formed on the
surface. With the growth of the copper content in the
sample, the experimental activation energy of dehy-
drogenation falls linearly (Fig. 6), due to the formation
of new centers on the samples with high contents of
deposited copper.
imental activation energies of dehydrogenation Е and log-
а
arithms of pre-exponential factor lnN : (1) initial activity,
0
(
2) repeated experiment
W300, %
W380, % Ea, kJ/mol
lnN0
Cu,
wt %
No.
1
2
1
2
1
2
1
2
1
2
3
4
5
0
1
3
1
7
8
7
–
1
8
8
57
57 –5.1 –5.1
To determine the effect preliminary reduction
treatment had on copper-containing samples, we
investigated the catalytic activity of the sample with a
copper content of 5% that was not subjected to prelim-
inary reduction (sample 5*). It was found that its
activity was 2–3 times lower than that of preliminarily
reduced samples. There was, however, considerable
28 35 32 90 220 2.5 32.8
40 40 47 90 114 1.9 8.8
5
27 43 56 87
17 58 76 134 –0.9 13.5
16 84 54 0.2 –7.8
94 2.2 4.7
5*
С Н
–
2
2
4
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ETHANOL DEHYDROGENATION ON COPPER CATALYSTS
2375
the surface of zirconia thus act as centers of both dehy-
W380, %
lnN0
60
dration and dehydrogenation (the values of Е for
Ea
а
dehydration and dehydrogenation are similar: 84 and
250
7
6 kJ/mol, respectively). During the reaction, how-
2+
ever, Cu ions are partially/fully reduced, leading to
the almost complete deactivation of the dehydration
centers and a rise in the number of dehydrogenation
50
40
30
20
200
centers (a rise in lnN ).
0
150
The nature of the dehydrogenation centers on the
sample not reduced differed from those on the sample
after preliminary reduction: the activation energy and
number of dehydrogenation centers for sample 5* are
lower than those of preliminarily reduced sample 5.
The activation of the unreduced copper sample under
the effect of the reaction medium was due to the for-
mation of a large number of catalytic active sites (a rise
in the exponential factor) with weaker bonding of the
alcohol to the surface.
100
50
10
0
0
1
3
Cu
5
X , %
Fig. 6. Effect of copper content in a sample on the charac-
teristics of the catalytic activity in repeated experiments: E
a
CONCLUSIONS
is the dehydrogenation activation energy (kJ/mol), W is
the conversion of ethanol at T = 380°C (%), and ln N is
0
The deposition of copper on tetragonal zirconia
allows us to improve its activity by 100–400%, and to
alter the selectivity of the conversion of ethanol,
depending on the pretreatment conditions. On
the logarithm of the pre-exponential factor.
autoactivation during catalysis, due to the reduction of
copper ions by reactive hydrogen, which is more effi-
cient than molecular hydrogen (Fig. 7). Along with
dehydrogenation, dehydration reaction with the eth-
ylene formation occurred on this sample. On the ini-
tial surface, the conversion of alcohol to ethylene
reduced copper-containing Cu/ZrO samples, only
2
acetaldehyde is formed in the alcohol dehydrogena-
tion reaction, while two products are formed on unre-
2+
duced Cu /ZrO samples: olefin and aldehyde. Close
2
E values for Cu/ZrO copper samples with different
a 2
reached 16% at 380°C, and the selectivity toward contents of copper, obtained for the initial surface of
dehydration at T = 380°C was 50%. In repeated exper- the catalysts, testify to the similar nature of the active
iments, the yield of ethylene did not exceed 2% of the sites that form during reduction treatment. During a
total alcohol conversion. Copper ions deposited on catalytic reaction, the nature of the centers in samples
W, %
60
50
40
30
20
Aldehyde
Aldehyde
Olefin
2
Olefin
1
10
1
2
0
2
50
300
350
T, °C
2+
Fig. 7. Temperature dependence of the conversion of ethanol to aldehyde and olefin on unreduced Cu /ZrO on (1) the initial
2
surface and (2) in the repeated experiment.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 12 2016

2376
CHUKLINA et al.
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