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More precisely, the band located at lꢁ295 nm grows in inten-
sity, most likely from light olefins produced and trapped on
the catalyst owing to the low diffusion properties of the mate-
mation of methoxy species is observed. Interestingly, during
the reaction period the depletion of the methoxy species
matches nicely with the formation of alkyl aromatics, suggest-
ing a relationship between them. To corroborate our finding
and rule out an effect of the temperature in the desorption
and/or depletion of the methoxy species, isothermal reactions
were run at different temperatures. The corresponding areas of
[
16,22,24]
rial at this stage.
In addition, the band at lꢁ410 nm
continues to increase in intensity during the deactivation
period while the band at l=345 nm starts to disappear, which
also suggests that the l=345 nm band most likely originates
from the reaction intermediates in olefin production. Further-
more, the band at l=600 nm assigned to PA species grows
during the whole reaction period of the material until reaching
a maximum when the catalyst is fully deactivated at 653 K,
which implies that the formation of this highly conjugated spe-
cies may play an important role in the deactivation of the cata-
lyst, more specifically by gradually reducing the accessibility
properties of the material until complete blockage. This is in
agreement with other lines of thought in which deactivation is
attributed to the formation of PA species inside the micropo-
ꢀ
1
the n˜ =940 and 1380 cm bands are plotted in Figure S2.
These results clearly confirm our analysis, namely that a de-
crease in the amount of methoxy species can be related to an
increase in the formation of alkyl aromatics.
The results in Figures 3c and S2 can be explained by a block-
age of or decrease in the accessibility of the zeotype cavities
by the alkylated PA species and a consequent decrease in the
formation of methoxy species. Once the methoxy species are
completely depleted, at approximately 673 K, the catalyst is fi-
nally deactivated. These findings point out that the methoxy
species might be directly involved in the catalytic process of
the formation of methylated aromatics, most likely by the
mechanisms described in the literature for the methylation of
[
25]
rous structure of the material. The origin of the deactivating
species, such as PA carbocations, is suggested to be mainly
[2]
from the reactions involving methanol instead of products.
A
[21a,30]
[31]
theoretical modeling study suggested that the side-chain
methylation of active hydrocarbon pool species might be a de-
activating route leading to coke precursors rather than olefin
olefins
and aromatics. However, at this stage in our re-
search we are unable to directly link the alkylation of aromatics
via methoxy species and further investigations are needed to
substantiate this point. Nevertheless, a direct link between the
accessibility of the material and the amount of methoxy spe-
cies can be made.
[
26]
production.
From the operando IR spectra, depicted in Figure 3b, one
can see several changes with respect to the spectra taken
during the induction period. In the OH stretching region, there
is a red shift in the band with increasing time on stream,
which could owe either to an interaction of the acid sites with
the hydrocarbons created in the catalyst or to a change in the
properties of the acid sites during the course of the reaction.
Surprisingly, despite the fact that protonated methyl benzenes
were observed by UV/Vis spectroscopy, no characteristic signa-
tures of those species were observed in the CH stretching/
bending and C=C stretching regions of the spectrum. This
could owe to the low concentration of such species. Above
We now draw our attention to the last stage of the reaction
in which the catalyst wafer is fully deactivated. We observed
changes in the UV/Vis operando spectra, displayed in Fig-
ure 4a, with temperature even though the catalyst is not
active for olefin production in the MTO reaction. Particularly,
the absorption band at l=600 nm, ascribed to highly methy-
lated PA species, gradually disappears with increasing reaction
temperature, whereas the absorption band at l=410 nm ap-
pears to increase in intensity. As none of the species can dif-
fuse outwards, these results can be explained by the transfor-
mation of the methylated PA species absorbing light at l=
600 nm into other species, such as monoaromatics, naphtha-
lenes, and olefins, by cracking reactions at such high tempera-
tures. Formation of the monoaromatics or less-methylated
naphthalene species is observed in the increase in intensity of
the absorption band at l=410 nm. In addition, the overall ab-
sorbance of the UV/Vis region increases after complete deacti-
vation of the catalyst, as can be better observed in the color-
coded Figure 1b. The increase in the overall absorbance is as-
cribed to the formation of highly conjugated aromatic species
that are likely unable to fit in the microporous structure of the
material and are consequently deposited onto the external sur-
623 K, changes in the shape of the CH stretching region are
observed and signatures ascribed to methoxy species are pro-
gressively eroded. Then, a new group of bands appears, with
ꢀ
1
a very intense absorption band at 2925 cm . This can be as-
cribed to CH stretching modes of methyl groups on PA species
[
27]
as suggested previously and in agreement with the UV/Vis
data. More indications of the formation of highly conjugated
ꢀ
1
aromatics can be obtained from the n˜ =1700–1400 cm
region, in which two new contributions from C=C stretching
ꢀ
1 [16,28]
modes appear at n˜ =1600 and 1580 cm ,
in addition to
ꢀ
1
the band at n˜ =1380 cm ascribed to CH bending modes
[
29]
from methyl groups on aromatics.
[17a]
One of the intriguing questions in the MTO chemistry is the
role of the methoxy species in the reaction. Aiming for
a better understanding of the function of such species, we in-
vestigate whether the depletion of methoxy species is linked
to the formation of methylated aromatics. By plotting the inte-
face of the material.
In the operando IR spectra, depicted in
Figure 4b, no substantial changes are observed in the OH
stretching region. On the contrary, the effect of temperature in
the CH stretching region is remarkable at this stage, with a de-
crease in the overall absorbance in this region if the reaction
temperature is higher than 653 K, which suggests a loss in the
methylation degree of the aromatic and PA species. Simultane-
ꢀ
1
grated area of the n˜ =940 and 1380 cm band in the IR spec-
tra with the reaction temperature, the methoxy species and
alkyl aromatics can be monitored, respectively. This plot is pre-
sented in Figure 3c; during the induction period only the for-
ꢀ
1
ously, a new band at n˜ =3015 cm grows in intensity, which is
likely due to the ring CH stretching vibration from less-alkylat-
ꢁ
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