Effect of iodine-containing promoter on the catalytic activity of silver in the course of ethylene glycol oxidation

Article abstract of DOI:10.1134/S1070427206090138

Surface morphology and catalytic properties of electrolytic silver crystals in partial oxidation of ethylene glycol into glyoxal were studied.

Full text of DOI:10.1134/S1070427206090138

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2006, Vol. 79, No. 9, pp. 1458 1462. Pleiades Publishing, Inc., 2006.
Original Russian Text V.S. Shmotin, A.S. Knyazev, A.I. Titkov, A.N. Salanov, O.V. Vodyankina, L.N. Kurina, 2006, published in Zhurnal Prikladnoi
Khimii, 2006, Vol. 79, No. 9, pp. 1474 1478.
CATALYSIS
Effect of Iodine-Containing Promoter on the Catalytic Activity
of Silver in the Course of Ethylene Glycol Oxidation
V. S. Shmotin, A. S. Knyazev, A. I. Titkov, A. N. Salanov,
O. V. Vodyankina, and L. N. Kurina
Tomsk State University, Tomsk, Russia
Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, Russia
Received March 3, 2006
Abstract Surface morphology and catalytic properties of electrolytic silver crystals in partial oxidation of
ethylene glycol into glyoxal were studied.
DOI: 10.1134/S1070427206090138
Glyoxal is widely used in industry for preparing
high-quality paper, films, adhesives, various polycon-
densates, and pharmaceuticals [1]. Being the nearest
analog of formaldehyde, glyoxal surpasses it in chemi-
cal properties. Partial oxidation of ethylene glycol on
polycrystalline silver catalysts, performed in the con-
tinuous flow mode, is the most ecnomically feasible
and facile procedure for preparing glyoxal. In this
case, the alcohol conversion is as high as 90%, and
the selectivity with respect to glyoxal reaches 55
60% [2, 3]. Promotion of the surface or introduction
of certain additives into the reaction mixture with
the subsequent treatment of the catalyst are the best
ways to enhance the activity of the catalytic systems.
Various promoters are used in catalysts for partial
oxidation of organic compounds, e.g., alkali metals
[4] and phosphorus-containing compounds [5]. The
promoting effect of halogen-containing hydrocarbons
on the partial oxidation of methanol to formaldehyde
in the presence of silver catalysts was studied in [6,
7]. It was found that, with addition of microamounts
of iodine-containing promoters into the reaction mix-
ture, the alcohol conversion and yield of formaldehyde
increase, whereas the yield of the products of deep
oxidation decreases. Taking into account that the proc-
esses of partial oxidation of methanol and ethylene
glycol are largely similar, in this study we examined
the catalytic synthesis of glyoxal in the presence of
ethyl iodide promoter.
by electrolysis of a KNO + NaNO + AgNO melt in
3 3 3
an open electrolysis bath using rotating cathode [8].
The crystal surface was finely cleaned by the proce-
dure described elsewhere [9]. The catalyst was pre-
pared by pressing of silver crystals with parafor-
maldehyde and subsequent annealing in an air flow at
650 C for 3 h.
The catalytic activity of silver was studied in a
flow catalytic installation with a fixed catalyst bed by
the procedure described in [10]. The height of the
catalyst bed was 20 mm, and the reactor diameter was
16 mm. The gaseous and liquid products obtained
in the course of oxidation were analyzed chromato-
graphically [11, 12].
The surface morphology of the silver samples was
studied using a JEOL 6460LV field-emission high-
vacuum scanning electron microscope (energy of the
primary electron beam 20 keV). To improve the
image contrast, the sample surface was coated with
gold (film thickness up to 0.05 m).
Data on the partial oxidation of ethylene glycol
into glyoxal in the absence of ethyl iodide and with its
addition to the reaction mixture in relation to the
oxygen content in the reaction mixture are plotted in
Fig. 1a. The molar content of the promoter added
was 6 ppm with respect to ethylene glycol. The oxy-
gen/ethylene glycol (O : EG) molar ratio was varied
2
from 0.8 to 1.8. As seen, with increasing oxygen con-
tent in the promoter-free reaction mixture the alcohol
conversion increases (Fig. 1a, curve 1), whereas the
selectivity of the glyoxal formation passes through a
maximum (Fig. 1a, curve 3). Addition of the promoter
into the reaction mixture improves the selectivity with
respect to the desired product in a broader range of
EXPERIMENTAL
Silver crystals were prepared at the Institute of
High-Temperature Electrochemistry (Ural Division,
Russian Academy of Sciences, Yekaterinburg, Russia)
1458

EFFECT OF IODINE-CONTAINING PROMOTER
1459
(c)
(a)
, S, %
T, K
O2 : EG
(b)
CO2, CO, %
(d)
T, K
O2 : EG
Fig. 1. (1, 3, 5, 7) Catalytic activity of silver catalyst without iodine-containing promoter and (2, 4, 6, 8) with addition of
promoter into the reaction mixture as a function of the (a, b) O : EG molar ratio and (c, d) temperature T. ( ) (1, 2) Conversion
2
of ethylene glycol; (S) (3, 4) selectivity with respect to glyoxal; yields of (5, 6) CO and (7, 8) CO.
2
the compositions of the reaction mixture as compared
to the initial sample (Fig. 1a, curve 4).
increasing temperature of treatment of the silver cata-
lyst with the promoted reaction mixture, the selec-
tivity with respect to glyoxal decreases (Fig. 1c,
curve 4), whereas the conversion of ethylene glycol
remains essentially constant (Fig. 1c, curve 2). This
decrease in the selectivity with respect to glyoxal is
probably due to the increasing rate of the iodine de-
sorption from the catalyst surface and rearrangement
of the centers of deep oxidation.
It should be noted that, with addition of the iodine-
containing promoter into the reaction mixture, the
yields of the products of deep oxidation decrease
(Fig. 1b), which is probably due to changes in the
chemical composition of the silver crystal surface.
Atomic iodine formed by the ethyl iodide decomposi-
tion is adsorbed on the silver surface and, occupying
the adsorption centers, decreases adsorption of the
oxygen atoms responsible for the deep oxidation [13].
This assumption is confirmed by certain decrease in
the ethylene glycol conversion in the presence of the
promoter (Fig. 1a, curve 2).
In the case of promoted reaction mixtures, the
yields of the products of deep oxidation are lower than
those in the promoter-free systems at the same tem-
peratures (Fig. 1d). These data suggest that, in the
presence of the promoter, the process is stable in the
broader range of experimental conditions. On the
whole, the silver catalyst in the presence of the iodine-
containing promoter is characterized by higher yield
The effect of temperature on the activity of the
silver catalyst in the course of ethylene glycol oxida-
tion is illustrated in Figs. 1c and 1d. As seen, with
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1460
SHMOTIN et al.
promoter content is insufficient for the selective con-
version, i.e., at low concentrations of the promoter in
the reaction mixture significant amounts of oxygen are
retained on the catalyst surface in the form of surface
,
%
s
oxide Ag O, which can participate in further oxida-
2
tion of the final product by the following reaction:
C2H2O2 + 3Ag2O
2CO2 + H2O + 6Ag.
S,
%
On the other hand, at molar ratios of ethyl iodide to
ethylene glycol higher than 6 ppm, the catalyst activ-
ity decreases due to poisoning of both the oxidation
centers on the silver surface participating in the deep
oxidation and oxygen-containing centers of the O
type participating in the selective oxidation of ethyl-
ene glycol into glyoxal. Participation of the O centers
of adsorbed oxygen in the glyoxal formation was
confirmed in [14].
C2H5I, ppm
Fig. 2. Catalytic activity of silver as a function of the
promoter (C H I) concentration in the reaction mixture
2
5
(molar ratio to ethylene glycol). Composition of the reac-
tion mixture: EG : O : N : H O 1.0 : 1.0 : 10.0 : 3.4,
2
2
2
T 560 C. ( ) (1) Conversion and (S) (2) selectivity.
Upon prolonged treatment of the catalyst with the
reaction mixture containing ethyl iodide in amounts
greater that 6 ppm, the process characteristics deterio-
rate owing to poisoning of the active centers on the
silver surface. We studied the possibility of the re-
generation of catalyst after prolonged treatment with
the iodine-containing promoter. For this purpose, the
spent catalyst was annealed in an air flow at 600 C.
After this treatment, the experiments on the catalytic
oxidation were repeated; the results are listed in the
table.
of the desired product and high stability in a wide
range of experimental conditions.
The dependence of the catalytic activity of silver
on the content of the iodine-containing promoter in
the reaction mixture is shown in Fig. 2. As seen
(Fig. 2, curve 1), the ethylene glycol conversion de-
creases with increasing promoter content in the reac-
tion mixture. This is probably due to the fact that
iodine competing with oxygen in the adsorption on
the silver surface decreases the content of adsorbed
atomic oxygen which provides selective oxidation of
ethylene glycol. Also, poisoning of the catalyst sur-
face with iodine due to excess removal of oxygen
cannot be excluded.
As seen from the table, annealing in an air flow at
600 C ensures almost complete regeneration of the
activity of the silver catalyst. This is probably due to
the iodine desorption from the sample surface and re-
generation of the centers of alcohol selective oxida-
tion. Thus, we found that the catalyst activity can be
enhanced by adding an iodine-containing promoter
(ethyl iodide) and the spent silver catalyst can be
readily regenerated.
The dependence of the selectivity with respect to
glyoxal on the promoter concentration passes through
a maximum, which is observed at a molar ratio of
ethyl iodide to ethylene glycol of about 6 ppm (Fig. 2,
curve 2). This is probably due to the following
reasons. At a molar ratio smaller than 6 ppm, the
An electron-microscopic examination was per-
formed to study the mechanism of promotion of the
iodine-containing additive and the effect of the reac-
tion mixture containing ethyl iodide on the mor-
phology of the silver catalyst. The surface of the ini-
tial sample is shown in Fig. 3a. The samples prepared
by electrolysis of the silver nitrate melt are elongated
particles (5 7 m wide and 40 60 m long). The
particle surface is smooth and well-rounded along
the edges. The structure of the sample surface is deter-
mined by the side topochemical processes occurring
Catalytic activity of the silver samples before and after
regeneration; composition of the reaction mixture:
EG : O2 : N2 : H2O 1.0 : 1.0 : 10.0 : 3.4, T 560 C
Ethylene glycol
conversion, %
Selectivity to
glyoxal, %
Sample
Ag
Ag/I
Ag/I:
90
80
55
66
prolonged treatment
after regeneration
70
80
45
64
during electrolysis of the molten mixture of KNO ,
3
NaNO , and AgNO [15].
3
3
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EFFECT OF IODINE-CONTAINING PROMOTER
The surface morphology of the silver catalysts
1461
strongly changes after treatment with both initial and
promoted reaction mixtures (Figs. 3b and 3c). As
seen, after treatment with the initial reaction mixture
EG : O : N : H O = 1.0 : 1.0 : 10.0 : 3.4 at 873 K,
2
2
2
channels 0.2 1.0 m in diameter are formed on the
silver catalyst surface (Fig. 3b), and their amount
upon treatment in the presence of the iodine-contain-
ing promoter is significantly smaller (Fig. 3c). It was
found [14, 16] that these channels are formed in the
near-surface silver layers in the course of treatment
due to the adsorption of C-containing reagents provid-
ing accumulation of the oxocarbon structures able to
diffuse into the sample bulk with increasing tempera-
ture. Under the action of the oxygen-containing reac-
tion mixture, the oxocarbon fibers formed in the near-
surface layer are burnt out, causing formation of
channels.
(a)
1 m
1 m
1 m
1 m
5 m
(b)
In accordance with the electron-microscopic data,
prolonged contact of the catalyst with the reaction
mixture leads to the formation of carbon deposits
(particles about 2 m long and 0.5 m thick, Fig. 3d).
This effect is due to the adsorption of carbon-contain-
ing reagents participating in the catalytic process [17].
Addition of ethyl iodide does not cause changes in the
silver crystal morphology (Fig. 3e). No pronounced
carbonization was observed after treatment under the
conditions similar to that of the initial sample.
(c)
(d)
(e)
Moreover, we observed almost no adsorption of
carbon structures, no formation of the carbonization
products, and no development of channels upon treat-
ment of silver with the iodine-containing reaction
mixture. This is most probably due to a decrease in
the content of oxygen adsorbed on the catalyst sur-
face, which directly participates in carbonization,
because no accumulation of the carbonization prod-
ucts on the silver surface is observed in the oxygen-
free systems [14].
In the reaction of ethyl iodide with the oxidized
silver surface, along with competitive adsorption,
removal of the surface oxygen is also possible by the
following reactions:
2Ag2O + 4C2H5I
G298
4AgI + 2C4H10 + O2,
564 kJ;
=
Fig. 3. Morphology of the silver catalyst surface: (a) initial
sample, (b) sample treated with the initial reaction mixture,
(c) catalyst treated with the promoted reaction mixture,
(d) sample treated with the initial reaction mixture for
a long period, and (e) sample treated with the promoted
reaction mixture a long period.
Ag2O + 2C2H5I
2AgI + 2C4H10O, G298 = 450 kJ.
In a study of the interaction of ethyl iodide with the
surface of a preoxidized silver single crystal, the reac-
tion products (butane, diethyl ether) used in the ther-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 79 No. 9 2006

1462
SHMOTIN et al.
mochemical calculations were detected by gas chro-
matography mass spectrometry [18].
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7. Dai, W.-L., Cao, Y., Deng, J., et al., Catal. Lett.,
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(1) The catalytic and physicochemical experiment-
al data showed that addition of an iodine-containing
promoter into the reaction mixture increases the selec-
tivity with respect to glyoxal by 5 10%.
8. Ivanovskii, L.E., Rozanov, I.G., Zotin, I.V., and Khra-
mov, A.P., Rasplavy, 1997, no. 2, pp. 51 69.
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Zh. Prikl. Khim., 2004, vol. 77, no. 1, pp. 43 47.
(2) The catalytic activity increases owing to a de-
crease in the contribution of the carbonization and
profound oxidation in the presence of the iodine-con-
taining additives.
10. Vodyankina, O.V., Kurina, L.N., Izatulina, G.A., and
Arkatova, L.A., Zh. Prikl. Khim., 1997, vol. 70,
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(3) Promoter decreases the concentration of the
centers of adsorbed oxygen responsible for the forma-
tion of carbonization products and occurrence of deep
oxidation of ethylene glycol. In this case, the amount
of oxygen participating in the selective conversion of
alcohol decreases only slightly, leading to only insig-
nificant decrease in the ethylene glycol conversion.
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