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Table 1 13C chemical shifts of surface species adsorbed on the H-MOR
zeolites obtained by DFT calculations and experiments
After 5 and 10 min respectively, the gas flow was stopped, and
liquid N2 was injected subsequently to cool the catalyst. The
obtained sample was then transferred into an NMR rotor in a
glove box. The corresponding 13C CP MAS NMR spectra are shown
in Fig. 3a and b. It is found that the carbonyl peak of MA in 8 MR
channels at d 185 ppm appeared earlier than the carbonyl signal
from the 12 MR channels at d 179 ppm. The results indicate that
carbonylation takes places in 8 MR, and then the MA diffuses
into 12 MR. These results demonstrate that the surface acetyl
species cannot be observed in ex situ continuous-flow experi-
ments, whereas the continuous-flow in situ 13C MAS NMR
spectroscopy enables direct observation of the intermediates.
Overall, the surface acetyl intermediates on H-MOR zeolites
(CH3CO-Z) were detected during DME carbonylation using
in situ continuous-flow 13C MAS NMR spectroscopy. The
ex situ experiments show that the carbonyl peak of MA in the
8 MR channels appeared prior to the carbonyl signal from
the 12 MR channels, indicating that the carbonylation reaction
likely takes place within the narrow 8 MR channels while the
broader 12 MR channels provide a channel for the transport of
reactants and products. This explains the much better activity
and selectivity of carbonylation on H-MOR than on other zeolite
containing 8 MR channels. These results also imply that the
carbonylation of methylated zeolites could initiate the for-
mation of a hydrocarbon pool in the MTO reaction.
d
13C calculated
d
13C from
experiments (ppm)
Species
by DFT (ppm)
Methoxyl
Acetyl
Methoxyl
Carbonyl
Methyl
Carbonyl
Methoxyl
Methyl
Carbonyl
Methoxyl
Methyl
Methoxyl
Methoxyl
62.6
169.2
26.1
181.7
55.3
22.7
188.4
60.3
26.1
63.5
—
58.4
169
34
179
54
24
185
54
MA in 12 MR
MA in 8 MR
24
63
52
DME
Methanol
experimental 13C NMR isotropic chemical shifts of the methyl and
carbonyl groups of methoxyl acetate adsorbed onto the Brønsted
acid sites of the zeolite (see details of DFT calculation in the ESI†).
The calculated 13C NMR chemical shifts for the surface acetyl
species are well consistent with the experimental results.
The signal at 169 ppm can be ascribed to the surface acetyl
species (CH3CO-Z Z for zeolites),15 which is consistent with the
theoretical calculations. The signal at 32 ppm might be attributed
to the methyl group of the surface acetyl species. As an unstable
intermediate, acetyl species can be subsequently captured by
water to produce acetic acid, by methanol or dimethyl ether to
give MA, or by ammonia to yield acetamide as reported by earlier
research studies. The acetyl species bound to an oxygen atom of
the catalyst has been reported to be the intermediate in the
carbonylation of methanol on the 12-tungstophosphoric acid
catalyst (12-H3PW12O40), based on the appearance of 13C MAS
NMR signals at d 13C(CO) = 193 ppm and d 13C(H3C–) = 22 ppm
attributed to the CH3–CO–(O–W) species.20 We did not observe
the surface acetyl species by the ex situ experiments (Fig. 3),
because it is quite unstable at room temperature.
This work was supported by the Ministry of Science and
Technology of China (2012CB224806). The authors would like
to thank Prof. X. L. Pan and Prof. F. Yang from Dalian Institute
of Chemical Physics for their help in paper compilation.
Notes and references
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An additional ex situ continuous-flow experiment was carried out
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Fig. 3 Ex situ continuous-flow 13C CP MAS NMR spectra after the gas mixture
has been injected into the reactor at 453 K for (a) 5 minutes and (b) 10 minutes,
respectively, recorded at room temperature and at a spinning rate of 12 kHz.
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