Mendeleev Commun., 2017, 27, 72–74
Table 1 Groups of studied catalysts.
Catalyst Molecular Catalyst Molecular
is favourable for complete oxidation. Therefore, to reduce an
undesired DMOE complete oxidation, this product should be
removed from the reactor as quickly as possible. Dilution of the
initial mixture with inert gas (He) and filling the free space of
the reactor with quartz can be a good solution of the problem.
Similar plots displaying dependences of the ODDME effi-
ciency on oxygen concentration in the feed gas were obtained for
the SrO–SnO and BaO–SnO catalysts. The highest selectivity
group
composition
group
composition
I
CaO–SnO (1:1)
II
10% Ga O /SiO
2
2
2
3
SrO–SnO (1:1)
5% In O /SiO
2 3 2
2
BaO–SnO (1:1)
5% In O + 5% La O /SiO
2 3 2 3 2
2
2
2
8
6
4
to DMOE for these catalysts was ~63–65%, and the highest yields
of DMOE on BaO–SnO and SrO–SnO were 5.5% and 6.5%,
1
2
2
2
3
respectively. Sufficiently high efficiency in ODDME was obtained
with the DME:air ratio close to 1:1, which corresponded to the
DME:O molar ratio of 4.6:1. This value appeared to be optimal
2
2
0
for ODDME under the applied reaction conditions. Calculations
show that this ratio corresponds to 70% conversion of oxygen.
We also studied the dependence of the ODDME reaction on
the contact time, which was changed by variation of flow rates of
DME, air and He, whereas the molar ratios between gases were
maintained constant (see Online Supplementary Materials). The
results show that the plot for the yield of DME has extreme
character, while the selectivity to DMOE sharply diminishes with
the increase of the contact time. The optimal contact time of
1–1.5 s provides the highest yield of DMOE, which most likely is
caused by the occurrence of two competing reactions, ODDME
and complete oxidation. Indeed, the higher contact time, the
higher probability for both reactant DME and product DMOE to
undergo complete oxidation. On the contrary, low contact times
shorten the interaction of DME and DMOE with molecular
oxygen preventing their complete oxidation.
2
00
250
300
350
T/°C
Figure 1 Temperature dependence of the DMOE yield: (1) CaO–SnO2,
2) SrO–SnO , and (3) BaO–SnO . Reaction conditions: 1 g of catalyst,
2 2
GHSV of DME, air and He is 860, 860 and 750 h , respectively.
(
–
1
metal in the catalyst, as well as on the reaction temperature. As it
was mentioned above, the DMOE formation is accompanied by
complete oxidation of DME, so that the yield of CO might be
2
comparable with the yield of DMOE. In addition, minor
quantities of methanol, formaldehyde and methyl formate were
also detected in the reaction products.
As it can be seen, elevation of the reaction temperature enhances
the efficiency of the catalysts, the highest activity being observed
at 310–325°C. The CaO–SnO2 sample displays the ultimate
activity in ODDME among the other tested catalysts, although the
activity of each catalyst gets on the temperature plateau when it
The conversion of DME into DMOE on using the catalysts
containing the variable valence metal oxides of group III was
evaluated. These oxides are often used in catalytic systems
destined for various oxidation reactions, in particular for methane
reaches ~320°C. On using CaO–SnO , the highest yield of DMOE,
2
6
~
8%, is obtained and raising the temperature from 250 to 320°C
causes increase in selectivity of DMOE formation, from 52% to
1%. This positive effect is probably resulted from the increase
oxidative dehydrogenation. We tested the following catalysts
on the basis of the silica supported metal oxides of group III:
10% Ga O /SiO , 5% In O /SiO , 5% In O , 5% La O /SiO . In all
6
2
3
2
2
3
2
2
3
2
3
2
in desorption rate of DMOE from the catalyst surface that in turn
reduces DMOE subjection to complete oxidation.
the cases, the experiments were carried out under optimal condi-
tions found above. All these catalysts are capable of catalyzing
ODDME. However, their specific activity in ODDME and selec-
tivity for DMOE formation differ (Figure 2).
Thus, the obtained results show that the nature of the alkaline-
earth metal in the catalyst affects the rate of ODDME. The higher
basicity of the oxide, the lower the catalyst efficiency in the
ODDME reaction. In this respect, the greatest difference between
the catalysts is observed when the reaction temperature does not
exceed 300°C. In fact, only traces of DMOE are detected over
According to Figure 2, the catalyst activity decreases in the
order: 5% In O , 5% La O /SiO > 5% In O /SiO > 10% Ga O /
2
3
2
3
2
2
3
2
2 3
SiO > CaO–SnO > SrO–SnO > BaO–SnO . Activity of the
2
2
2
2
supported catalysts is much higher than those of co-precipitated
ones. The highest activity was exhibited by 5% In O , 5% La O /
BaO–SnO catalyst at 250°C, while the yield of DMOE reaches
2
2
3
2
3
~
3% over CaO–SnO catalyst under similar reaction conditions.
SiO catalyst, which makes it possible to suppose a synergism
2
2
It was revealed that catalyst efficiency in the ODDME reaction,
between indium and lanthanum oxides at their equal content in the
catalyst. Moreover, the increase in the catalyst activity is accom-
panied by the increase in selectivity of the reaction. Apparently,
it is caused by the competition between two reactions occurring
in particular over CaO–SnO , depended on the concentration
2
of oxygen in the initial gas mixture (see Online Supplementary
Materials). Apparently, there should be an optimal concentration
of oxygen in the inlet gas flow providing the catalyst best per-
formance. Indeed, the increase of the air flow rate leads, on the
one hand, to the growth of oxygen concentration on the catalyst
surface, which should be beneficial for ODDME. On the other
hand, this should decrease the contact time, which diminishes
the yield of DMOE. In our experiments, when air flow rate is less
6
5
4
3
2
1
0
Specific activity 80
Selectivity
6
0
40
–1
than 8–10 ml min the selectivity is at the 65–67% level, whereas
the selectivity falls to the 40% level and even lower at the air
flow rates higher than 10 ml min–1 (GHSV = 750 h ).
2
0
0
–1
To evaluate the role of the gas-phase reactions in DME con-
version, several experiments were carried out with the no-load
reactor. For this purpose a small quantity of DMOE was added to
the initial gas mixture. It was found that DMOE can also participate
in the free space complete oxidation, with this reaction rate being
higher than the rate of DME complete oxidation. Obviously,
slow desorption of DMOE from the catalyst into the gas phase
Figure 2 Specific activity in ODDME and selectivity to DMOE under
optimal conditions for the catalysts.
–
73 –