Alkane activation initiated by hydride transfer: Co-conversion of propane and methanol over h-ZSM-5 Zeolite

Article abstract of DOI:10.1002/anie.201501248

Co-conversion of alkane with another reactant over zeolite catalysts has emerged as a new approach to the long-standing challenge of alkane transformation. With the aid of solid-state NMR spectroscopy and GC-MS analysis, it was found that the co-conversion of propane and methanol can be readily initiated by hydride transfer at temperatures of ≥449K over the acidic zeolite H-ZSM-5. The formation of ¹³C-labeled methane and singly ¹³C-labeled n-butanes in selective labeling experiments provided the first evidence for the initial hydride transfer from propane to surface methoxy intermediates. The results not only provide new insight into carbocation chemistry of solid acids, but also shed light on the low-temperature transformation of alkanes for industrial applications.

Full text of DOI:10.1002/anie.201501248

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201501248
German Edition: DOI: 10.1002/ange.201501248
Reaction Mechanisms
Alkane Activation Initiated by Hydride Transfer: Co-conversion of
Propane and Methanol over H-ZSM-5 Zeolite**
Si-Min Yu, Jian-Feng Wu, Chong Liu, Wei Liu, Shi Bai, Jun Huang, and Wei Wang*
Dedicated to Professor Michael Hunger on the occasion of his 60th birthday
Abstract: Co-conversion of alkane with another reactant over
zeolite catalysts has emerged as a new approach to the long-
standing challenge of alkane transformation. With the aid of
solid-state NMR spectroscopy and GC-MS analysis, it was
found that the co-conversion of propane and methanol can be
readily initiated by hydride transfer at temperatures of ꢀ 449 K
the development of robust catalytic systems for efficient
activation and further conversion of alkanes is severely
hindered by the lack of mechanistic understanding. In this
regard, solid-state NMR spectroscopy has been applied as
a powerful tool for monitoring the mechanistic events on
[
7]
heterogeneous catalysts. In addition, GC-MS analysis on the
1
3
13
2
over the acidic zeolite H-ZSM-5. The formation of C-labeled
distribution of either C or H labels in the reaction products
may offer the complementary information on many mecha-
nistic details. Through solid-state magic-angle-spinning
(MAS) NMR spectroscopy and GC-MS analysis, we found
herein, that upon addition of methanol, the CÀH bond
1
3
methane and singly C-labeled n-butanes in selective labeling
experiments provided the first evidence for the initial hydride
transfer from propane to surface methoxy intermediates. The
results not only provide new insight into carbocation chemistry
of solid acids, but also shed light on the low-temperature
transformation of alkanes for industrial applications.
activation of propane was initiated by hydride transfer to
surface methoxy species over zeolite H-ZSM-5 (Scheme 1).
Initiated by hydride transfer, even at temperature as low as
A
lkanes are the main components in natural gas and crude
oil. The conversion of abundant but inert alkanes into high-
value-added products has been a long-standing challenge to
[
1]
[2]
chemists. The direct transformation of alkanes suffers
from the inertness of CÀH/CÀC bonds and unfavorable
thermodynamics which result in high reaction temperature,
low product yield/selectivity, and unrealistic cost for industrial
applications. Alternatively, the co-conversion of alkanes with
[
3]
[4]
Scheme 1. C
ÀH bond activation of alkane (RÀH) initiated by hydride
reactive hydrocarbons or oxygenates over zeolites has
recently been explored to overcome these limitations. Meth-
anol is a key platform chemical produced from various
sources and can be readily converted over acidic zeolites
transfer to surface methoxy intermediate (denoted as CH -ZSM-5 in
3
this work) on acidic zeolites. Once formed by the initial hydride
+
transfer, the carbenium ion (R ) can be readily converted into value-
added products through secondary reactions such as methylation by
surface methoxy species (see Scheme 2).
[5]
through the industrial process. Many efforts have accord-
[
6]
ingly been attempted to use methanol as the co-reactant for
[
6a]
[6b]
[6c]
coupling with methane,
ethane,
C –C4 alkanes,
n-
3
[
6d]
[6e]
[6f]
butane, n-hexane, and petroleum naphtha. However,
449 K, the co-conversion of propane and methanol can be
further achieved by secondary reactions, such as propene
methylation (Scheme 2). This work not only details the long-
pursued evidence for hydride transfer over zeolite catalysts,
but also offers insightful information on practical utilization
of alkanes through the co-conversion strategy.
[*] S.-M. Yu, Dr. J.-F. Wu, C. Liu, Dr. W. Liu, Prof. Dr. S. Bai,
Prof. Dr. W. Wang
State Key Laboratory of Applied Organic Chemistry, College of
Chemistry and Chemical Engineering, Lanzhou University
Lanzhou, Gansu 730000 (P. R. China)
and
Collaborative Innovation Center of Chemical Science and Engi-
neering, Tianjin 300071 (P. R. China)
To obtain unambiguous evidence for the initial hydride
1
3
transfer, we chose C-labeled methanol and nonlabeled
propane (as the model compound of inert alkane) for the
selective labeling experiments. Surface methoxy species
E-mail: wang_wei@lzu.edu.cn
(
Scheme 1) have been consistently identified as the reactive
Dr. J. Huang
[8]
intermediates in methanol conversion over acidic zeolites,
and previous studies have demonstrated its role as an
Laboratory for Catalysis Engineering, School of Chemical and
Biomolecular Engineering, The University of Sydney
New South Wales 2006 (Australia)
[
9]
effective methylating agent. Herein, we discovered a new
reactivity for suface methoxy species, that is, the hydride
abstraction in alkane activation on zeolite H-ZSM-5. The
[**] This work was financially supported by the National Natural Science
Foundation for Distinguished Young Scholars of China (No.
[
9]
21425206) and the 973 Program of China (2012CB821701).
stopped-flow protocol has allowed us to isolate reactive
intermediates and study its reactivity by in situ solid-state
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201501248.
1
3
MAS NMR spectroscopy. By using this protocol, the C-
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spectra recorded for the reactions occurring at T= 449–471 K.
1
3
The dominant C MAS NMR signal at d = 60 ppm is
1
3
[10,11]
attributed to CH -ZSM-5.
Meanwhile, the signal at
3
d = 52 ppm arises from terminal methoxy species or strongly-
[10]
bonded methanol. The NMR signals at d = 16 and 17 ppm
belong to the methyl and methylene carbon atoms, respec-
tively, of propane. Upon the reaction at 449 K for 5 hours
(
Figure 1a), a new NMR signal appeared at d = À7 ppm and
results from the formation of the adsorbed methane
1
3
[12]
[
CH (a)]. The fitted GC-MS data of the isotope distribu-
4
tion patterns (Figure 1a, right side) showed that methane was
1
3
13
almost 100% C-labeled, thus indicating that CH4 was
Scheme 2. The role of surface methoxy species in the co-conversion of
13
[13]
derived from CH -ZSM-5.
With the increase of the
3
methanol and propane on zeolite H-ZSM-5: initiation and methylation.
heating time and temperature (Figure 1b–d), the formation
1
3
The reaction is initiated by hydride transfer from propane to CH3-
13
1
3
13
of methane [ CH (a), at d = À7 ppm] became more evident,
4
ZSM-5, through which CH is formed. The singly C-labeled n-
4
and was accompanied by the appearance of free methane
butanes are formed through propene methylation as one of the
secondary reactions depicted herein. The symbol * denotes the C-
1
3
[12]
1
3
[ CH (g), at d = 11 ppm].
Again, GC-MS analysis (Fig-
4
1
3
labeled carbon atoms.
ure 1b–d) indicates that methane was 100% C-labeled. It is
worth mentioning that propane or CH -ZSM-5 alone on
1
3
3
zeolite H-ZSM-5 was not reactive under the same reaction
conditions (see the Supporting Information). These results
demonstrated that surface methoxy species can abstract
a hydride from propane to form methane. This conclusion
was further supported by the reaction of deuterated surface
methoxy species (CD -ZSM-5, prepared from CD OH on H-
1
3
labeled surface methoxy species (denoted as CH -ZSM-5)
3
[
10]
13
was accordingly prepared
from CH OH. The reaction
between nonlabeled propane (chemical purity of 99.99%)
and CH -ZSM-5 could then be monitored by both solid-state
C MAS NMR spectroscopy and GC-MS analysis.
A key result obtained is that the C-labeled methane was
formed as the primary product. Figure 1 (left side) shows the
C high-power proton decoupling (HPDEC) MAS NMR
3
1
3
3
1
3
3
3
1
3
ZSM-5) with propane. The corresponding formation of CD H
3
was verified by the GC-MS analysis (see Figure S8 in the
Supporting Information).
1
3
Upon the initial hydride transfer
1
3
from propane to CH -ZSM-5, la-
3
beled methane and the adsorbed
+
C H7 carbenium ion should be
3
simultaneously formed (Scheme 2).
1
3
Given that it is C-unlabeled and
readily involved in the secondary
reactions, the C -species formed ini-
3
tially from propane could not be
1
3
directly observed by C MAS NMR
spectroscopy. However, the analysis
of secondary reaction products pro-
vided the supplemental mechanistic
information. As shown in Figure 1,
besides methane, n-butane was also
observed, as evidenced by the
1
3
C NMR signals at d = 26 (methyl-
ene group) and 14 ppm (methyl
[
14]
group). Importantly, GC-MS anal-
ysis (Figure 1b–d) verified that the
1
3
C-labeled n-butane products could
1
3
13
only have a single C label ([ C ]-n-
1
butane, see also Table S1), thus indi-
cating that the labeled n-butanes
came from the C-methylation of
1
3
13
Figure 1. C HPDEC MAS NMR spectra (left) and the corresponding bar graphs (right; for
methane and n-butane products, respectively) of the mass spectra in molecular ion region,
nonlabeled C species. Meanwhile,
3
1
3
13
recorded upon the reaction of C-labeled surface methoxy species ( CH -ZSM-5, coverage of ca.
3% Brønsted acid sites on 60 mg of H-ZSM-5) and nonlabeled propane [n( CH -ZSM-5):n-
3
13
3
the selective formation of [ C ]-n-
1
3
1
1
(
butane rather than isobutane is in
excellent agreement with those
propane)ꢁ1:9] at 449 K for 5 h (a), 449 K for 10 h (b), 471 K for 5 h (c), and 471 K for 10 h (d).
Each reaction was conducted in a sealed glass insert to avoid the escape of gaseous products, and
the glass insert was directly inserted into a 7 mm rotor for solid-state NMR measurements. The
reported for propene methylation
1
3
[15]
symbol * denotes the C-labeled carbon atoms.
over acidic zeolites.
Thus, the
2
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1
3
observation of singly C-labeled n-butane indicated one of
that the CÀH bond activation of propane can be indeed
subsequent transformations after the initial hydride transfer:
the adsorbed C H7 carbenium ion (i.e., surface isopropoxy
species) is deprotonated to produce the C nonlabeled
propene, which can be methylated subsequently by CH3-
ZSM-5 (Scheme 2). Propene methylation on the acidic
zeolites involves the protonated methylcyclopropane as the
intermediate or transition state. Opening the cyclopropane
ring would result in the formation of the singly C-labeled n-
butanes ([ C ]-n-butane at either methyl or methylene
position), which have also been identified by solid-state
NMR and GC-MS. Upon further reaction at 471 K for
0 hours (Figure 1d), other C-labeled products (such as
isobutane: d = 25 ppm and additional propane: d = 16 and
7 ppm) were produced, thus implying the occurrence of
complex secondary reactions (see Figure S5 and Section H in
the Supporting Information for detailed discussions).
The experimental observation of CH and singly C-
labeled n-butanes provided direct evidence for the hydride
transfer process over acidic zeolite H-ZSM-5 (Scheme 2).
To highlight the role of hydride transfer to surface methoxy
species for propane activation, we compared the reaction of
propane alone (Figure 2a) with that of propane in the
presence of surface methoxy species (Figure 2b) by GC-MS
initiated by hydride transfer to surface methoxy species.
Hydride transfer, as an important step in acid-catalyzed
transformation of hydrocarbons, is directly related to the
initial CÀH bond activation of alkanes and to the formation of
+
3
1
3
[
15]
13
[
1b]
the corresponding carbocations.
Once the carbocation is
formed, it can be readily converted into the final products by
[15]
cracking, skeletal isomerization, alkylation, etc. (Scheme 1).
1
3
[19]
In analogy to the classical cases of superacids,
hydride
1
3
transfer has long been proposed to occur between alkanes and
surface alkoxy species (as the adsorbed carbenium ions) over
1
[
20]
[17]
acidic zeolites. However, this elementary step has never
been experimentally verified because of the complexity of the
secondary reactions. Herein, the isolation of surface methoxy
intermediates by a stopped-flow protocol and the selective
labeling experiments made it possible to verify the initial
hydride transfer at low temperatures through a combination
of solid-state NMR spectroscopy and GC-MS. This exper-
imental evidence will certainly bridge the gap between the
mechanistic studies of alkane activation over liquid and solid
acids.
1
3
1
1
1
3
13
4
[16]
Very similar results were obtained upon the co-conversion
of methanol and propane over zeolite H-ZSM-5 (see Fig-
[21]
ure S12). The practical implication is that methanol may
indeed work as an ideal co-reactant for alkane transforma-
[6]
tion at low temperatures. For example, the CMHC (coupled
methanol-hydrocarbon cracking) process over zeolite H-
[
6g]
ZSM-5 was initially proposed to achieve energy compen-
sation, but the catalytic results showed that the conversion of
[22]
hydrocarbon products is also increased. Our mechanistic
study not only explains this phenomenon, but also suggests
a new possibility for practical conversion of inert alkanes at
low temperatures.
1
3
In summary, we have used solid-state C MAS NMR
spectroscopy and GC-MS analysis together to study the
initiation mechanism for the co-conversion of propane and
1
3
methanol over zeolite H-ZSM-5. The formation of C-
1
3
labeled methane and the singly C-labeled n-butanes was
accordingly identified. These results show that, with methanol
as the co-reactant, propane conversion can be readily initiated
by hydride transfer and followed by methylation on zeolite H-
ZSM-5, both of which involve surface methoxy species as the
key intermediate (Scheme 2). This research also suggests that
co-conversion with methanol may indeed work as an alter-
native approach to alkane activation. We expect that the in-
depth understanding of these elementary steps occurring over
acidic zeolites will therefore inspire further investigation on
low-temperature conversion of alkanes for industrial utiliza-
tion.
Figure 2. Normalized GC-MS data for the reactions on zeolite H-ZSM-
5
of nonlabeled propane alone (a) and of nonlabeled propane together
1
3
with CH -ZSM-5 (b). The reactions were conducted in parallel at
3
4
71 K for 10 h, and the samples were measured with the identical GC-
MS parameters.
analysis of the gaseous components. Upon heating at 471 K
for 10 hours, propane alone (Figure 2a) was unreactive over
[
13]
zeolite H-ZSM-5. On the contrary, upon the co-reaction of
1
3
CH -ZSM-5 and propane under the identical reaction
3
1
3
conditions (Figure 2b), 100% C-labeled methane, together
with the singly C-labeled n-butane and isobutane, were
1
3
formed in a large quantity (see Table S1). Furthermore, we
Keywords: alkanes · hydride transfer · NMR spectroscopy ·
reaction mechanisms · zeolites
1
3
conducted a kinetics investigation on the reaction of CH3-
1
3
ZSM-5 and propane by in situ C MAS NMR spectroscopy at
T= 449–471 K (see Figures S9–S11). The apparent activation
À1
energy of about 139 kJmol was derived from an Arrhenius
plot of methane formation rates against the reaction temper-
[17]
atures. This value is reasonably lower than the apparent
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Received: February 9, 2015
Published online: && &&, &&&&
4
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Reaction Mechanisms
1
3
Put a label on it: C-labeled methane and
1
3
singly C-labeled n-butanes were identi-
S.-M. Yu, J.-F. Wu, C. Liu, W. Liu, S. Bai,
fied as the primary products from the
1
3
J. Huang, W. Wang*
&&&& — &&&&
initial reaction of propane and CH OH
3
over the zeolite H-ZSM-5. This result
provides experimental evidence for the
proposed hydride transfer between alka-
nes and surface alkoxy species over acidic
zeolites, and also suggests a new possi-
bility for alkane transformation at low
temperatures.
Alkane Activation Initiated by Hydride
Transfer: Co-conversion of Propane and
Methanol over H-ZSM-5 Zeolite
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!