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relationship between ethene and lower methylbenzene has
route for the formation of ethene in the MTO reaction over
zeolite H-ZSM-5.
[8c]
been proposed on Beta zeolite and the typical MTO catalyst
[
24]
SAPO-34 as well, although the exact undertaking mechanism
is not well known. There is no doubt that the carbocations
play a crucial role in the MTO reaction over acidic zeolite cata- Experimental Section
lysts. The observation and characterization of persistent carbo-
À1
Material and catalytic reactions: Methanol (WHSV=2 h ) was re-
acted over H-ZSM-5 (0.3 g, Si/Al=15) in a fixed bed reactor at
a temperature range of approximately 275–4008C. In each case,
the catalysts were compressed to wafers that were crushed and
sieved to obtain 60–80 mesh particles, and then the particles were
activated in place prior to the reaction by heating at 4008C in
flowing helium for 1 h. The methyl- or ethyl-substituted cyclopen-
tadienes were synthesized according to previously reported proce-
cations has considerably deepened our understanding of the
MTO chemistry. Recently, cyclic carbocations such as the 1,3,4-
trimethylcyclopentenyl cation and the pentamethylbenzenium
ion were simultaneously observed and identified in the MTO
reaction as the key intermediates to produce propene on H-
[
11b]
ZSM-5.
The formation and stabilization of special carboca-
tions in the channels or cages of zeolites strictly depends on
the spatial and chemical environment where the carbocations
reside. Bulky carbocations such as both the heptamethylbenze-
nium cation and the pentamethylcyclopentenyl cation can be
produced from repeat methylation of aromatics in zeolite SSZ-
[26].
dures.
For the reaction of the ethylcyclopentadienes, a pulse-
quench reactor was used to quench the reaction by reducing the
reaction temperature with liquid nitrogen within a very short
[2d]
period (<1 s). Typically, when the reaction proceeded in a pulse-
quench reactor for a pre-set period, the reaction was thermally
quenched by pulsing liquid nitrogen onto the catalyst bed, which
was achieved by using high-speed valves controlled by a GC com-
puter. In each case, an aliquot of the reactant (10 mL) was pulsed
into the reactor (heated at 3508C) containing H-ZSM-5 (0.3 g) and
allowed to react for 8 s, before quenching by liquid nitrogen. The
effluent and the trapped products were determined by GC-MS
analysis.
1
3, which has the same topology structure as SAPO-34 (having
2
[13b]
a three-dimensional cage of ca. 6.712 ).
Theoretical cal-
culations suggested a feasible side-chain route for the forma-
tion of propene and ethene through the heptamethylbenzeni-
[
25]
um ion. In comparison, the geometric constraints imposed
2
by medium pore (sinusoidal channel: 5.55.1 , straight chan-
2
nel: 5.35.6 ) of H-ZSM-5 only allows pentamethylbenzeni-
12
13
13
In the C/ C methanol isotope transient experiments, C natural
um ions to be present as the persistent species. Moreover, as
abundance methanol was fed at 3508C for 30 min before switching
[11b,14]
13
13
indicated by theoretical calculations,
the pentamethylben-
to C-labeled methanol (99% C) and was allowed to react for
1
3
zenium ions favor the formation of propene and isobutene on
H-ZSM-5. Although bulky methylbenzenes (e.g., pentamethyl-
benzene) that cannot diffuse from the channel of H-ZSM-5 are
formed in the MTO reaction, the higher reaction barriers in-
volved make it difficult to form olefins like ethene through
a variable period up to 2.5 min. The evolution of the C compo-
nents in the effluent was determined by GC-MS analysis after the
13
C methanol switch at 0.5, 1, and 2 min. The corresponding iso-
topic data for the trapped species in the zeolite channels during
the reaction were obtained.
[9]
GC and GC-MS analysis: The effluent products were analyzed
quantitatively by online GC (Shimadzu GC-2010 plus) equipped
with a flame-ionization detector and a fused silica capillary column
Supel-Q PLOT (30 m, 0.32 mm i.d., 15 mm film thickness). The tem-
perature programming started at 358C (maintained for 1 min), fol-
a side-chain route on the basis of methylbenzenes. On the
contrary, the contraction of the C ring of aromatics into small-
6
er cyclic C ethylcyclopentenyl cations provides an alternative
5
to the aromatics-based hydrocarbon-pool route for the forma-
tion of ethene. Our experimental results unambiguously dem-
onstrated that the aromatics-based paring route is viable for
the formation of ethene through the ethylcyclopentenyl cation
intermediates in the MTO reaction over zeolite H-ZSM-5.
À1
lowed by a rate of 58Cmin to a final temperature of 2008C
(maintained for 30 min).
The catalyst with the trapped products was dissolved in 20 wt%
HF solution and then the solution was extracted with CH Cl . The
2
2
bottom layer containing the organic phase of the extracted solu-
tion was separated and analyzed by gas chromatograph.
Conclusion
The isotopic compositions of the retained compositions were ana-
lyzed by GC-MS (Shimadzu GCMS-2010 plus) equipped with
a fused silica capillary column Petrocol DH 100 (100 m, 0.25 mm
i.d., 0.5 mm film thickness). The following temperature program-
In summary, by using NMR spectroscopy and GC-MS, we inves-
tigated the methanol to olefins conversion over the H-ZSM-5
zeolite. Strong evidence was found for ethyl-substituted cyclo-
pentadienyl species being intermediates in the ethene forma-
tion on H-ZSM-5 during the MTO reaction. The stable ethylcy-
clopentenyl cations 1, 2, and 3 as key intermediates were for
the first time experimentally observed and identified in the
MTO reaction. The distinct reactivity of the ethylcyclopentenyl
cations was linked to lower methylbenzenes (xylene and trime-
thylbenzene) and ethene. All these compounds can be inte-
grated in a full catalytic cycle working through a paring mech-
anism in which the ethylcyclopentenyl cation acts as the criti-
cal hydrocarbon-pool species splitting off an ethyl group as
ethene product. The results presented here shed new insight
into the hydrocarbon-pool chemistry and provide a viable
ming was applied: maintained at an initial temperature of 508C for
À1
1
min, followed by a rate of 108Cmin to a final temperature of
2508C (maintained for 20 min).
Solid-state and liquid-state NMR analysis: The MTO reaction was
13
carried out by reacting C-labeled methanol over H-ZSM-5 (0.3 g)
for 30 min at 3508C. After the reaction was quenched by liquid ni-
trogen, the reactor containing the catalyst sample was sealed. The
sealed reactor was transferred to a glovebox filled with pure N2
and the catalyst sample containing the trapped species was
packed into to an NMR rotor for NMR measurements. All solid-
state NMR experiments were acquired on a Bruker-Avance III-800
spectrometer at 18.8 T, equipped with a 3.2 mm probe, with reso-
1
13
nance frequencies of 800.20 and 201.23 MHz for H and C, respec-
13
1
tively. Single-pulse C MAS experiments with H decoupling were
12067 ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2015, 21, 12061 – 12068