Direct observation of cyclic carbenium ions and their role in the catalytic cycle of the methanol-to-olefin reaction over chabazite zeolites

Article abstract of DOI:10.1002/anie.201303586

Carbenium ions in zeolites: Two important carbenium ions have been observed for the first time under working conditions of the methanol-to-olefins (MTO) reaction over chabazite zeolites using ¹³C NMR spectroscopy. Their crucial roles in the MTO

Full text of DOI:10.1002/anie.201303586

.
Angewandte
Communications
DOI: 10.1002/anie.201303586
Reactive Intermediates
Direct Observation of Cyclic Carbenium Ions and Their Role in the
Catalytic Cycle of the Methanol-to-Olefin Reaction over Chabazite
Zeolites**
Shutao Xu, Anmin Zheng, Yingxu Wei, Jingrun Chen, Jinzhe Li, Yueying Chu, Mozhi Zhang,
Quanyi Wang, You Zhou, Jinbang Wang, Feng Deng, and Zhongmin Liu*
[
10–15]
The diminishing reserve of crude oil and the rapidly increas-
ing demands for the base chemicals, such as ethene and
propene, provide strong driving force for exploring alterna-
tive chemical feedstocks and developing non-petrochemical
processes. To date, the methanol-to-olefins (MTO) process
has been one of the most successful non-petrochemical routes
for the production of light olefins from abundant resources of
produce olefins.
Specifically, the paring mechanism
involves the contraction of six-membered ring cations (poly-
methylbenzenium cations) and the expansion of five-mem-
bered ring cations (polymethylcyclopentenyl cations). In
contrast, the side-chain methylation route proceeds via the
methanol methylation on polymethylbenzenium cations and
subsequent elimination of side-chain groups to produce
olefins.
Direct observation of these carbenium ions involved in
the proposed mechanism under real conditions is of great
significance to understand the reaction mechanism and the
structure–performance correlation of the MTO reaction.
Even though some carbenium ions in the MTO reaction,
such as cations 1–4 (see Scheme 1) have been identified using
in situ solid-state NMR, the formation of these carbenium
[
1–3]
natural gas or coal.
products of the MTO reaction in the global energy chain and
Considering the essential role of the
[4]
chemical industry, investigations of the MTO reaction
mechanism are crucial for both of the fundamental science
and industrial application. Catalysts used in the MTO process
are typically microporous solid acids, including zeolites and
zeotype molecular sieves, among which the H-ZSM-5 zeolite
with framework type MFI and SAPO-34 with framework type
CHA deliver the best catalytic performance in the MTO
[
1–4]
reaction.
past 30 years,
bond in the MTO process remains elusive. In general, the
Despite the tremendous research efforts over the
[
4–8]
the reaction mechanism of the first CÀC
[
9]
hydrocarbon pool (HCP) mechanism, that is, that cyclic
organic species confined in the zeolite cage or intersection of
channels act as co-catalysts, has been generally accepted as
a rational explanation for the olefins production from the C
1
Scheme 1. The series of carbenium ions observed with in situ NMR
spectroscopy in zeolites: 1: 1,1,2,4,6-pentamethylbenzenium cation; 2:
[2]
reactant, methanol. Two reaction routes have been pro-
posed to explain the MTO reaction pathway according to the
HCP mechanism, namely the side-chain methylation route
and the paring route. In both of the two routes, carbenium
ions are involved and act as the important intermediates to
1
,3-dimethylcyclopentenyl cation; 3: heptamethylbenzenium cation
+
+
(heptaMB ); 4: heptamethylcyclopentenyl cation (heptaMCP ); 5:
+
pentamethylcyclopentenyl cation (pentaMCP ).
[
+]
[
*] Dr. S. Xu, Prof. Dr. Y. Wei, J. Chen, Dr. J. Li, M. Zhang, Q. Wang,
ions in the catalyst was verified mostly indirectly or by the aid
of the reaction of co-feeding methanol and benzene or
Y. Zhou, J. Wang, Prof. Dr. Z. Liu
National Engineering Laboratory for Methanol to Olefins
Dalian National Laboratory for Clean Energy
Dalian Institute of Chemical Physics
Chinese Academy of Sciences, Dalian 116023 (P. R. China)
E-mail: liuzm@dicp.ac.cn
[
16–19]
methylbenzene.
Owing to the difficulty in the direct
observation of these HCP species, understanding of their
formation and significance in methanol conversion over
zeolites or SAPO catalysts under real working conditions is
still a huge challenge. Until now, only cation 2 on H-ZSM-5
and cation 3 on DNL-6 with framework type RHO were
observed directly under real working conditions using meth-
[
+]
Prof. Dr. A. Zheng, Y. Chu, Prof. Dr. F. Deng
State Key Laboratory of Magnetic Resonance and
Atomic Molecular Physics, Wuhan Center for Magnetic Resonance
Wuhan Institute of Physics and Mathematics
[
17,20]
anol as the only reactant.
H-SAPO-34 and H-SSZ-13 with
Chinese Academy of Sciences, Wuhan 430071 (P. R. China)
+
framework type CHA are the very important catalysts for the
MTO process owing to their great success or potential in the
[
] These authors contributed equally to this work.
[
**] We are grateful for the financial support of the National Natural
[
2,10]
industrial applications.
However, still no experimental
Science Foundation of China (No. 21073228, 21173255, 21103180,
clue was reported towards the carbenium ions identifications
over these two catalysts, in turn, the corresponding reaction
pathways for the MTO reaction over them are remain
unknown to date.
2
1273230, and 21273005). We gratefully thank Dr. Fan Yang for
fruitful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303586.
1
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Herein, using the multiple techniques, we report the
observations of six-membered (cation 3) and five-membered
cation 5) ring cations under the real working conditions of
190–210 ppm. The appearance of the resonance peaks at 240–
1
3
255, 155, and 56 ppm in the C MAS NMR spectra implies
the formation of five-membered ring cations (polymethylcy-
(
[17,19]
methanol conversion. Their simultaneous formation was
identified for the first time in the H-SSZ-13 and H-SAPO-
clopentenyl cations) over the two catalysts,
while for H-
SSZ-13, apart from these signals from polymethylcyclopen-
tenyl cations, the appearance of the peaks at 203, 190, and
144 ppm also confirms the formation of six-membered ring
cations. According to the previous studies carried out over H-
1
3
3
4 using methanol as the only reactant with C MAS NMR.
The cation 5 in particular is a new carbenium ion observed in
the MTO reaction. The assignments of the two carbenium
ions were unambiguously confirmed by the combined tech-
niques of NMR, GC-MS, and theoretical calculation. Fur-
thermore, on the basis of the experimental results, methanol
conversion mechanisms (that is, side-chain methylation and
paring mechanisms) with the two carbenium ions as the
reaction intermediates were theoretically explored in more
detail.
[
18]
[20]
Beta
and DNL-6
catalysts, the benzenium cation
observed here is ascribed to heptamethylbenzenium (hep-
+
taMB ), which will be demonstrated later by theoretical
1
3
calculations of the C chemical shift (Supporting Informa-
tion, Table S1). Therefore, for the first time, in the catalysts
with chabazite topologies, the most important reaction
intermediates, polymethylcyclopentenyl cations and polyme-
thylbenzenium cations, were successfully captured and
directly observed under real MTO reaction conditions.
For a precise structure determination, confined organic
Structural characterizations of H-SSZ-13 and H-SAPO-34
are described in the Supporting Information, Figures S1–S3.
The strategy to capture and observe the carbenium ions
involves mild reaction conditions to lower their reactivity and
appropriate catalytic environment for their accommodation
and stabilization, such as the cage structure and acid environ-
ment of the catalysts. The catalytic performances of methanol
conversion over H-SSZ-13 and H-SAPO-34 at a low temper-
ature range of 275–4008C are shown in the Supporting
1
2
species in H-SSZ-13 after a parallel CH OH reaction were
3
[24]
analyzed by GC-MS (Supporting Information, Figure S5).
A peak at retention time of 10.4 min was ascribed to
pentamethylcyclopentadiene (pentaMCP), indicating that
the observed five-membered ring carbenium ions confined
in CHA cages and observed with C MAS NMR is pentam-
ethylcyclopentenyl cation (pentaMCP ). The deprotonation
1
3
[
21]
+
Information, Figure S4 and reported in our previous study.
1
3
+
Figure 1 shows representative C MAS NMR spectra of H-
process from pentaMCP to pentaMCP is shown in the
Supporting Information, Scheme S1.
1
3
On the basis of accurate C chemical shift calculations
Supporting Information, Figure S6, Table S1), the theoretical
(
1
3
+
C chemical shift of heptaMB cation (at 198, 141, and
+
1
85 ppm) and pentaMCP cation (at 245, 150, and 58 ppm)
inside H-SSZ-13 are determined and in good agreement with
NMR observations. Pure pentaMCP dissolved in 98% H SO4
2
(Supporting Information, Figure S7) and adsorbed in H-Beta
zeolite (Supporting Information, Figure S8), methods to
+
[25]
produce pure pentaMCP ,
show very close values in
chemical shifts to the results in Figure 1. Therefore, the
+
+
formations of heptaMB and pentaMCP were consolidated
by combining the NMR and GC-MS techniques and theoret-
ical prediction.
1
3
Figure 1. C MAS NMR spectra of a) retained organic species in H-
+
Notably, for H-SAPO-34, only pentaMCP has been
1
3
SAPO-34 after continuous-flow CH OH reaction at 3008C for 15 min
and b) retained organic species in H-SSZ-13 after continuous-flow
CH OH reaction at 2758C for 25 min. The asterisk denotes spinning
3
observed, while over H-SSZ-13, the generation of both
+
+
1
3
heptaMB and pentaMCP was shown. Based on the
extensive experimental attempts and the assistance of theo-
retical calculations, Nicholas et al. predicted that the carbe-
nium ions can be persistent inside zeolites if their deproton-
ated neutral hydrocarbons have a proton affinity (PA) value
3
1
3
+
side-bands. Insets: calculated C chemical shifts of pentaMCP and
+
heptaMB ions inside the H-SSZ-13 zeolite (the optimized structures
of the two carbenium ions confined in H-SSZ-13 are shown in the
Supporting Information, Figure S6).
À1 [12]
more than 209 kcalmol . The theoretically predictions at
MP2/6-311G(d,p) level indicate that the PA value of hexam-
ethylmethylenecyclohexadiene (HMMC, the deprotonated
SAPO-34 and H-SSZ-13 catalysts during methanol conver-
+
sion at 2758C. Diamondoid hydrocarbons, such as methyl-
form of heptaMB ) and pentaMCP are 239.3 and 223.5–
[
22]
À1
adamantanes (10–50 ppm) and alkylated aromatics, such as
227.2 kcalmol , respectively (Supporting Information, Fig-
[
23]
methylbenzenes (120–140 ppm),
are the most predomi-
ure S9). According to the assumption of Nicholas et al., the
+
+
nantly formed species among the retained organics. The
adsorbed methanol at 50.5 ppm and dimethyl ether (DME) at
heptaMB and pentaMCP ions can exist inside both the H-
+
SAPO-34 and H-SSZ-13 zeolites. And heptaMB ion may be
more stable than pentaMCP owing to its larger PA value.
+
6
0.5 ppm can also be observed on H-SSZ-13 catalyst at 2758C.
Besides these stable organic species, even in low intensity,
some signals appear in the downfield region (140–250 ppm)
with the characteristic chemical shift at about 240–250 and
This is in good agreement with the observation in H-SSZ-13
that these two carbenium ions have been determined by our
C NMR experiment. However, it is not the case for the H-
1
3
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+
SAPO-34 over which only pentaMCP is observed. This
implies that PA value is not the only factor for estimating the
stabilities of the carbenium ions in a confined environment.
Considering their identical CHA cage for the carbenium ions
+
accommodation, the difference in heptaMB formation and
observation is possibly associated with acid strength of the
Brçnsted acid sites that are responsible for the protonation
and stabilization of the intermediates in the catalysts.
Stronger Brçnsted acidity of H-SSZ-13 than H-SAPO-34,
1
confirmed by H MAS NMR spectra, NH -TPD, and
3
1
3
13
C MAS NMR of [2- C]acetone adsorption (Supporting
Information, Figures S3, S10, S11), was considered to be the
+
reason of the stabilization of heptaMB in H-SSZ-13.
However, the PA value and acid strength cannot explain
+
+
why heptaMB ion is less stable than pentaMCP ion in the
CHA cage. Therefore, their reactivity during methanol
conversion should also be considered as well as their
generation and stabilization in acid catalysts. The methanol
reaction performed at various temperatures over H-SSZ-13
and H-SAPO-34 catalysts (Supporting Information, Figur-
+
es S12, S13) demonstrates that pentaMCP could be observed
in a wide temperature range, even at relatively high temper-
+
ature of 3508C, while heptaMB was only observed at low
temperature, such as 2758C, implying its extremely high
reactivity. More details about the activation energy barriers of
+
+
heptaMB and pentaMCP conversions will be theoretically
calculated and are discussed below.
1
3
13
Figure 2. a) C MAS NMR spectra of CH OH reaction on H-SSZ-13
3
with time on-stream under continuous-flow condition at 2758C.
Asterisk denotes spinning side-bands. b) Relationship between the
conversion of methanol and the concentrations of carbenium ions with
time on-stream.
^ total concentration of the two ions.
To further investigate the roles of the observed carbenium
1
3
ions in the MTO reaction, a series of tests of CH OH
3
+
+
conversion over H-SSZ-13 were performed at 2758C and
&
methanol conversion, ~ pentaMCP , ! heptaMB ,
1
3
stopped at various times on-stream. The C MAS NMR
spectra presenting the variation of confined organics are
+
shown in Figure 2a, and the concentrations of heptaMB and
+
+
12
pentaMCP were quantitatively calculated according to the
pentaMCP generated olefin products containing both
C
1
3
integral area of the ring carbon atoms with the characteristic
peaks at 189/190 ppm and 240–250 ppm, respectively. The
and C atoms (Figure 3; Supporting Information, S14),
suggesting that olefins products were formed via the inter-
+
+
13
12
concentration evolutions of heptaMB , pentaMCP and both
of the two carbenium ions with time on-stream (TOS)
action of [ C]-methanol and the confined [ C]-carbenium
ions. Moreover, the isotopic distribution of the confined
+
(
Figure 2b) are almost consistent with the methanol con-
pentaMCP, the deprotonated product of pentaMCP , indi-
1
3
version variation, except for the 20 minute delay of highest
cates that C atoms from labeled methanol are incorporated
+
+
concentration of heptaMB . This confirmed the participation
into pentaMCP during the MTO reaction (Figure 3). This
of the two cyclic carbenium ions in the MTO reaction as the
+
important intermediates. The observations of heptaMB and
+
pentaMCP and the correlation of their concentrations with
methanol conversion mean that both paring and side-chain
methylation mechanisms are possible for methanol conver-
sion over H-SSZ-13 zeolite with framework type CHA. Even
though the paring mechanism has not been proposed for
methanol conversion over eight-ring catalysts, it cannot be
ruled out on the base of the observation and confirmation of
+
pentaMCP .
+
Previous studies have proved that heptaMB is a very
important intermediate during the MTO reaction over H-
[
11,20]
Beta and DNL-6 catalysts.
In the present work, the
+
participation of pentaMCP as a hydrocarbon pool species in
MTO under real reaction condition were followed by [ C]-
methanol/[ C]-methanol switch experiments (for the detailed
procedure, see the Supporting Information). Feeding [ C]-
1
2
1
3
12
Figure 3. Isotopic distribution of pentaMCP formed in the [ C/
1
3
1
3
C]methanol switch experiments over H-SSZ-13 at 2758C. Inset: the
isotopic distribution of propene.
1
2
methanol upon the H-SSZ-13 catalyst with confined [ C]-
1
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Angewandte
Chemie
more important route for MTO reaction
for a detailed explanation, see the Sup-
(
porting Information, Section S2.5 and
Figure S15). This is in accordance with
the theoretical calculation results.
Based on the proposed reaction
cycles with carbenium ions as the impor-
tant intermediates, the observability of
+
+
the heptaMB and pentaMCP can be
discussed. As their generation is energeti-
cally favorable, the capture of them is
determined by their stability and lifetime,
which are closely related with the reac-
tion energy barriers of their further
transformation to other intermediates.
According to the catalytic cycle pre-
sented in Figure 4 (Supporting Informa-
tion, Figure S17), the transformation of
+
pentaMCP ions (M4/P) needs to over-
come relative higher energy barriers
À1
(
32.45 and 33.04 kcalmol ) than the
+
À1
heptaMB
ions
(17–23 kcalmol ).
Therefore, the generated pentaMCP
ion is more stable than heptaMB ion in
+
+
H-SSZ-13, and can be more readily
observed by the NMR method. This
trend is also in good agreement with our
Figure 4. Catalytic cycles of the paring and side-chain reaction mechanisms for the MTO
+
experimental results that heptaMB ions
+
+
conversion with the involvement of pentaMCP and heptaMB in H-SSZ-13 zeolite. The
À1
only can be observed at low temperature.
In conclusion, for the first time two
very important carbenium ions involved
calculated energy barriers are given in kcalmol
.
+
+
+
further proved the participation of pentaMCP in the
methanol conversion as the hydrocarbon pool species.
To clarify the prevailing reaction route of methanol
conversion, theoretical calculations were carried out to
evaluate the reaction pathway following the paring mecha-
nism and side-chain methylation mechanism with the involve-
ment of heptaMB and pentaMCP as the characteristic
intermediates for these two mechanisms (Figure 4; Support-
ing Information, Figure S16). For the two catalytic cycles, the
eliminations of propene from M3/P and M6/S (M3/P-M4/P
in MTO reaction, heptaMB and pentaMCP ions, have been
directly observed in CHA-type catalysts under the real
reaction conditions. The detection of the carbenium ions
depends on the acid catalytic environment and the reactivity
+
of these ions during the catalytic transformation. PentaMCP
+
is more readily to be observed than heptaMB in the CHA
+
+
nanocage owing to the higher activation energy required for
1
3
its further transformation. C MAS NMR measurements and
isotopic switch experiment provide substantial proofs that
+
+
both pentaMCP and heptaMB are important intermediates
for the MTO reaction. The evaluation of two catalytic cycles
following paring and side-chain methylation mechanisms
demonstrated that both are energetically feasible reaction
routes, while the side-chain methylation mechanism is more
predominant owing to the lower energy barrier.
and M6/S-M7/S steps in Figure 4), with activation energies of
À1
3
6.66 and 26.65 kcalmol , are the rate-determining steps for
the paring and the side-chain methylation mechanisms. In the
competitive reaction systems, the reaction usually follows the
pathway with low activation energy, so the side-chain
methylation mechanism may be more dominant in the MTO
reaction over H-SSZ-13 zeolite. But noticeably, the difference
in activation energy between the two mechanisms is not very
Received: April 27, 2013
Revised: July 22, 2013
Published online: September 13, 2013
À1
large (ca. 10 kcalmol ), parallel occurrence of the two
reaction cycles is also very possible in the MTO reaction.
PentaMCP (M4/P, see Figure 4) formation and accommoda-
+
Keywords: carbenium ions · density functional calculations ·
methanol · olefins · zeolites
.
tion in the CHA cage of zeolite catalyst, and their partic-
ipation in methanol conversion is the direct evidence of
paring mechanism with the five-membered-ring carbenium
involvement. Moreover, detailed isotopic distribution analy-
sis of the main olefin product, propene (Figure 3) indicates
that the contribution of paring mechanism for olefin gener-
ation is secondary, and side-chain methylation mechanism is
[
[
1] M. Stçcker, Microporous Mesoporous Mater. 1999, 29, 3 – 48.
2] J. Haw, W. Song, D. Marcus, J. Nicholas, Acc. Chem. Res. 2003,
36, 317 – 326.
[3] W. Wang, M. Hunger, Acc. Chem. Res. 2008, 41, 895 – 904.
Angew. Chem. Int. Ed. 2013, 52, 11564 –11568
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 11567

.
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Communications
[
4] U. Olsbye, S. Svelle, M. Bjorgen, P. Beato, T. Janssens, F. Joensen,
S. Bordiga, K. Lillerud, Angew. Chem. 2012, 124, 5910 – 5933;
Angew. Chem. Int. Ed. 2012, 51, 5810 – 5831.
5] K. Hemelsoet, J. Mynsbrugge, K. Wispelaere, M. Waroquier, V.
Speybroeck, ChemPhysChem 2013, 14, 1526 – 1545.
6] S. Ilias, A. Bhan, J. Catal. 2012, 290, 186 – 192.
7] H. Yamazaki, H. Shima, H. Imai, T. Yokoi, T. Tatsumi, J. Kondo,
Angew. Chem. 2011, 123, 1893 – 1896; Angew. Chem. Int. Ed.
2008, 120, 5257 – 5260; Angew. Chem. Int. Ed. 2008, 47, 5179 –
5182.
[16] T. Xu, D. Barich, P. Goguen, W. Song, Z. Wang, J. Nicholas, J.
Haw, J. Am. Chem. Soc. 1998, 120, 4025 – 4026.
[17] P. Goguen, T. Xu, D. Barich, T. Skloss, W. Song, Z. Wang, J.
Nicholas, J. Haw, J. Am. Chem. Soc. 1998, 120, 2650 – 2651.
[18] W. Song, J. Nicholas, A. Sassi, J. Haw, Catal. Lett. 2002, 81, 49 –
53.
[
[
[
2
011, 50, 1853 – 1856.
[19] W. Song, J. Nicholas, J. Haw, J. Phys. Chem. B 2001, 105, 4317 –
4323.
[
8] S. Kim, H. Jang, J. Lee, H. Min, S. Hong, G. Seo, Chem.
Commun. 2011, 47, 9498 – 9500.
9] I. Dahl, S. Kolboe, J. Catal. 1994, 149, 458 – 464.
[20] J. Li, Y. Wei, J. Chen, P. Tian, X. Su, S. Xu, Y. Qi, Q. Wang, Y.
Zhou, Y. He, Z. Liu, J. Am. Chem. Soc. 2012, 134, 836 – 839.
[21] Y. Wei, J. Li, C. Yuan, S. Xu, Y. Zhou, J. Chen, Q. Wang, Q.
Zhang, Z. Liu, Chem. Commun. 2012, 48, 3082 – 3084.
[22] T. Pehk, E. Lippmaa, V. Sevostjanova, M. Krayuschkin, A.
Tarasova, Org. Magn. Reson. 1971, 3, 783 – 790.
[23] W. Wang, Y. Jiang, M. Hunger, Catal. Today 2006, 113, 102 – 114.
[24] M. Guisnet, L. Costa, F. Ribeiro, J. Mol. Catal. A 2009, 305, 69 –
83.
[
[
10] B. Arstad, J. Nicholas, J. Haw, J. Am. Chem. Soc. 2004, 126,
991 – 3001.
11] M. Bjørgen, U. Olsbye, D. Petersen, S. Kolboe, J. Catal. 2004,
21, 1 – 10.
2
[
2
[
[
12] J. Nicholas, J. Haw, J. Am. Chem. Soc. 1998, 120, 11804 – 11805.
13] W. Song, J. Haw, J. Nicholas, C. Heneghan, J. Am. Chem. Soc.
2
000, 122, 10726 – 10727.
[
[
14] M. Bjørgen, F. Bonino, S. Kolboe, K. Lillerud, A. Zecchina, S.
Bordiga, J. Am. Chem. Soc. 2003, 125, 15863 – 15868.
15] D. McCann, D. Lesthaeghe, P. Kletnieks, D. Guenther, M.
Hayman, V. Speybroeck, M. Waroquier, J. Haw, Angew. Chem.
[25] J. Haw, J. Nicholas, T. Xu, L. Beck, D. Ferguson, Acc. Chem. Res.
1996, 29, 259 – 267.
1
1568 www.angewandte.org
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