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J. Li et al. / Catalysis Today 171 (2011) 221–228
channels) were easy to be blocked by the coke species, and this
might be responsible for the deactivation. The location of these
coke species in the catalyst was still unknown. Some researchers
[25,35–38] argued that ZSM-5 (MFI type with 10 member ring
and intersections) cannot provide enough spaces for the forma-
tion of coke species larger than tetramethylbenzenes, and this was
believed to be the reason of the long life time of ZSM-5 for methanol
conversion. The deactivation of ZSM-5 was caused by the coke on
the external surface of the zeolite crystals [35]. The diameter of
ZSM-22 channels was close to that of ZSM-5. So the formation of
the large coke species in the channels might also be suppressed. But
large coke species may form on the external surface or near the pore
mouth. These species could age to insoluble graphitic species or be
adsorbed in the channels (or near the pore mouth) and then blocked
the pore openings. Among the three catalysts, ZSM-22 has the high-
est selectivity for C5. Although the C6+ selectivity over ZSM-22 was
olefins (>C3) might be formed through the olefin methylation-
cracking route. This can also explain the extremely high propene
selectivity over SAPO-34 at the first methanol pulse. During the
induction period the active hydrocarbon pool species in SAPO-34
were relatively rare, and the reaction occurred more possibly fol-
lowed olefin methylation-cracking route. This was responsible for
the relatively higher propene selectivity obtained during the induc-
tion period.
The incorporation of 12C atoms into the products over ZSM-
5 was less obvious than that over SAPO-34, but ethene shows
appreciable incorporation of 12C atoms. These observations were
consistent with the work of Svelle and Bjorgen [24,25] and pre-
dicted that the olefin methylation-cracking mechanism was also
one of the main reaction routes for the formation of C3+ olefins over
ZSM-5. But ethene was formed mainly from the reaction following
+
comparable with that over ZSM-5, the composition was very dif-
+
ferent. Over ZSM-5 the aromatics constituted the main part of C6
while olefins dominated over ZSM-22.
,
conversion was even weakened by using ZSM-22 as catalyst. At
the 2nd to 4th injection of 13C methanol, the total 13C contents of
propene, butene, pentene and hexene were higher than 95%. As
After 15 pulses of 12C-methanol pre-injection, 13C methanol
pulse reaction was also conducted and the isotopic distribu-
tion indicated hydrocarbon products containing only 13C atoms
13C-containing products increased with the further 13C-methanol
injection. These observations were similar to those over ZSM-5 and
reflected the reaction over the two zeolites possibly followed a close
reaction mechanism.
+
shown in Table 3, C3 olefins were the main products of methanol
conversion over ZSM-22, it was reasonable to propose that olefin
methylation-cracking route is operative for methanol conversion
over ZSM-22. The incorporation of 12C atoms into ethene over ZSM-
+
22 was higher than that into C3 olefins. This suggested that over
ZSM-22 the ethene might be also formed mainly from hydrocarbon
pool mechanism, as which was formed over ZSM-5. However, the
very low selectivity for ethene was observed over ZSM-22 indicat-
ing methanol conversion with hydrocarbon pool mechanism was
suppressed.
Different from the publications [24,25], the isotopic switch
experiments in this study were performed on the pulse reaction
system. In this setup, the mixing of 12C-methanol and 13C-methanol
was avoided and the switch of 12C/13C could be clear-cut and
an immediate products analysis after isotopic switch could be
realized. This also makes it possible to correlate the differences
in the 13C distribution to the reaction mechanism. 12C-methanol
pre-reaction generated 12C-polymethylbenzenes in the cages or
the products would predict that these 12C-polymethylbenzenes
worked as the reaction center in methanol-to-olefin conversion,
and the reaction followed hydrocarbon pool mechanism [5,6].
Through another reaction route proposed for methanol conver-
sion, olefin methylation-cracking [24,25] without involving the
12C-polymethylbenzenes retained in the catalysts, the products
generation would be independent of the scrambling of 12C atoms.
However, if both of the two reaction mechanisms were allowed to
operate on a specific catalyst, the products from the former reac-
tion route would possibly serve as the initial olefins for the later
reaction route, which makes the reaction route determination more
difficult from the isotopic distribution. Anyway, the observations in
the above-mentioned 13C switch experiments are still helpful for
distinguishing the reaction route of methanol conversion over the
three catalysts, even the conclusion is difficult to be drawn exactly.
The total 13C contents in the products of methanol conversion
over different catalysts were calculated from the detailed isotopic
distribution and plotted in Fig. 4 against the pulse number of 13C
methanol. Among the three catalysts studied, 12C atoms incorpora-
tion is more predominant over SAPO-34 than ZSM-5 and ZSM-22,
especially in ethene. This indicates that the hydrocarbon pool
mechanism is the main reaction route over SAPO-34, especially for
ethene formation. In the report of [34], the authors performed the
reaction system, and they found that the 13C contents of ethene,
propene and butenes were very similar. However, in the present
work, ethene formation after 12C/13C switch presented evident dif-
ference in the 13C contents of products. As shown in Fig. 4, the total
13C content of ethene was always lower than that of other products.
This finding suggested that even over SAPO-34, part of the higher
The differences in the product distribution over ZSM-22 and
ZSM-5 can be explained by taking the zeolite topology into account.
ZSM-22 has only one dimensional 10-member ring channels with
2
˚
was 5.7 × 4.6 A . ZSM-5 also has the 10-member ring channels but
it contains channel intersections which can accommodate cyclic
species as hydrocarbon pool. Actually, large amount of toluene and
+
xylene were detected among C6 products over ZSM-5. However,
+
linear olefins accounted for the largest part of C6 over ZSM-22.
From the point of reaction mechanism, the channels of ZSM-22
sterically hindered the reaction following the mechanism of hydro-
carbon pool, and methanol conversion mainly go through the olefin
methylation and cracking route. This could explain the very low
selectivity of ethene over ZSM-22. For clarity, the reaction routes
of methanol conversion over the three zeolites with different pore
structures were illustrated in Fig. 5.
To further evaluate the importance of the above-mentioned
two catalytic cycles over these three catalysts, the co-reaction of
ordinary olefin (1-butene) or aromatic (p-xylene) with 13C labeled
13C-methanol was about 1/20. The co-reactions were carried out
at 450 ◦C with contact time of 0.04 s. In the co-reaction of 13C-
methanol and 1-butene, 1-butene was produced by 1-butanol in
situ. The co-reaction results were shown in Fig. 6. Over SAPO-34, the
13C contents of the effluents in the co-reaction of 13C-methanol and
12C-p-xylene were higher than 95%. By contrast, relatively low 13
C
content of the products (especially heavier olefins) was presented
in the co-reaction of 13C-methanol and 12C-butene over SAPO-34.
These differences indicated that the added 12C-p-xylene molecules
had no involvement in methanol conversion due to the larger
molecular size than the pore diameter of SAPO-34. The appearance
of 12C atoms from the added 12C-butene molecules in the prod-
ucts implied the participation of butene in the reaction and higher
olefins might be formed through the olefin methylation-cracking
reaction route, which is consistent with the work of pulse reaction
over SAPO-34.