Article
pubs.acs.org/JPCC
Methane Activation on In-Modified ZSM‑5 Zeolite. H/D Hydrogen
Exchange of the Alkane with Brønsted Acid Sites
Sergei S. Arzumanov,†,‡ Ilya B. Moroz,‡ Dieter Freude,∥ Jürgen Haase,∥ and Alexander G. Stepanov*,†,‡
†
Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk
630090, Russia
‡
Faculty of Natural Sciences, Department of Physical Chemistry, Novosibirsk State University, Pirogova Street 2, Novosibirsk 630090,
Russia
∥
Universität Leipzig, Fakultät für Physik und Geowissenschaften, Linnéstrasse 5, 04103 Leipzig, Germany
S Supporting Information
*
ABSTRACT: In relation to clarifying the pathway of methane
activation on In-modified zeolites, a comparative analysis of
kinetics of hydrogen (H/D) exchange between methane-d4
and Brønsted acid sites (BAS) for both the pure acid form
zeolite (H-ZSM-5) and In-modified zeolites (In+/H-ZSM-5
and InO+/H-ZSM-5) has been performed. Monitoring of the
kinetics has been carried out with 1H magic-angle spinning
NMR spectroscopy in situ within the temperature range of
453−568 K. While the rate of exchange on In+/H-ZSM-5 is 1
order of magnitude larger than that on H-ZSM-5, the exchange occurs on InO+/H-ZSM-5 by 2 orders of magnitude faster than
that on H-ZSM-5. Significant increase of the rate and decrease of the activation energy (Ea = 74 ± 6 kJ mol−1) and the
temperature threshold (453 K) for the reaction of the exchange on InO+/H-ZSM-5 compared to the rate, activation energy, and
temperature threshold (543 K) for the reaction on In+/H-ZSM-5 (Ea = 127 ± 27 kJ mol−1) and H-ZSM-5 (Ea = 118 ± 9 kJ
mol−1) have been rationalized in terms of involvement of both InO+ and BAS in activation of methane molecules on the zeolite.
Some transient intermediate complex of methane with the zeolite InO+ species and BAS has been assumed to be formed within
the zeolite pore. This complex is involved either in the reaction of H/D exchange with BAS of the zeolite or evolves further to
offer indium methyl species.
1. INTRODUCTION
Methane is the most naturally abundant but extremely inert
hydrocarbon. Therefore, its involvement in chemical transformation is a challenge for modern chemistry. Numerous
attempts have been made to overcome a chemical inertness of
methane with the use of heterogeneous catalysts, including
metal oxides1,2 and metal-modified zeolites.3,4 Methane has
been shown to be activated on In-modified zeolite ZSM-5 and
reacted with ethylene to obtain propylene.5,6 Some conclusions
on the mechanism of methane activation on In/H-ZSM-5 have
been recently made.7 In particular, the nature of surface
intermediates formed at methane interaction with indium
cationic species of the zeolite has been clarified.7 However, the
role of Brønsted acid sites (BAS) of In/H-ZSM-5 zeolite in
methane activation and the formation of the surface
intermediates remains unrevealed.
The hydrogen (H/D) exchange reaction between the
Brønsted acid sites of solid catalysts and the alkane molecules
precedes usually the alkane chemical transformation.8 This
reaction is often used to characterize the activation of alkanes
on the catalysts.9−25 The information on the features of
activation of alkane molecules can be derived by monitoring in
situ the H/D exchange reaction between alkane and BAS of
zeolite with 1H MAS NMR.19,26,27 The application of this
© 2014 American Chemical Society
method to the study of H/D exchange on zeolites modified
with Zn or Ga27−30 allowed us to establish the role of the metal
centers and BAS in the activation of C1−C4 alkanes on these
zeolites.
We have analyzed the kinetics of H/D exchange of methane
with Brønsted acid sites of zeolites H-ZSM- 5 modified with
indium (In+/H-ZSM-5 and InO+/H-ZSM-5) and compared
these kinetics with the kinetics on the pure acid form of zeolite
H-ZSM-5. This approach to the study of small alkane activation
on heterogeneous catalysts allowed us to establish some
peculiarities of methane activation by Brønsted acid site of
zeolite ZSM-5 modified by indium.
2. EXPERIMENTAL SECTION
2.1. Material Syntheses, Characterization, and Sample
Preparation. The hydrogen form of zeolite ZSM-5 (H-ZSM5) used in this work for the synthesis of the In-form zeolites
was prepared from industrially produced (Tricat Zeolites)
ammonium form of ZSM-5 by calcination at 773 K for 5 h. Inmodified zeolite (In/H-ZSM-5) was prepared by a reductive
Received: April 16, 2014
Revised: June 5, 2014
Published: June 5, 2014
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rapidly increased within 3−10 min by 40−100 K to the reaction
temperature and equilibrated for 4−5 min; then, the acquisition
of the NMR signal started.
solid-state ion-exchange reaction between indium(III) oxide
and H-ZSM-5 zeolite as described previously.7 Approximately
30% substitution of Brønsted acid sites of parent H-ZSM-5 for
indium species was reached by this procedure. 29Si magic-angle
spinning (MAS) NMR analysis31 revealed a silicon-toaluminum ratio of 17 for both the pure acid form and Inmodified H-ZSM-5 zeolite samples. Indium content was 7.9 wt
% in In/H-ZSM-5, which corresponded to indium loading of
8.0 wt % in the sample. 27Al MAS NMR spectra of the hydrated
H-ZSM-5 and In/H-ZSM-5 zeolites did not show any extra
framework aluminum atoms in these samples. Further, the
activation of the In/H-ZSM-5 sample (100 mg) was performed
in a glass tube under vacuum (10−3 Pa) at 673 K for 24 h.
Afterward, two samples with activated In/H-ZSM-5 were
prepared. Reduction of In/H-ZSM-5 in hydrogen (760 mbar)
at 773 K for 30 min and further evacuation at 673 K for 2 h
resulted in the sample, designated as In+/H-ZSM-5. The
second sample, denoted as InO+/H-ZSM-5, was first reduced
and evacuated using the method described for the previous
sample. However, it was then subjected to oxygen treatment
(760 mbar O2 at 623 K for 30 min) and evacuated at 673 K for
2 h. According to X-ray photoelectron spectroscopy (XPS) and
27
Al, 29Si, and 1H MAS NMR data,7 thus prepared In+/H-ZSM5 sample showed unit cell composition In01.3In+3.0H1.7Al4.7
Alnonfr0.7Si90.6O192 and contained indium in the form of In+
cations, whereas InO+ cations represented the main form of
indium in InO+/H-ZSM-5 sample, which exhibited the unit cell
composition (In2O3)0.65(InO+)3.0H1.7Al4.7Alnonfr0.7Si90.6O192.
Methane-d4 (99% D) used in this work was purchased from
Aldrich Chemical Co., Inc. Industrially produced oxygen and
hydrogen gases were used without further purification.
The zeolite sample (100 mg; H-ZSM-5, In+/H-ZSM-5, or
InO+/H-ZSM-5) located in axially high-symmetrical glass tube
of 5.5 mm outer diameter and 10 mm length was loaded under
vacuum with methane-d4 (300 μmol g−1). Afterward, the
sample located in the glass tube was sealed by flame. While it
was being sealed, the tube with the sample was kept in liquid
nitrogen, preventing heating of the sample by the flame. The
glass tube with the zeolite sample ideally fit NMR zirconia rotor
(7 mm outer diameter). This allowed rotation of the sealed
tube with the rate of 3000 Hz, which provided both the analysis
of the reaction products in situ and monitoring the kinetics of
H/D exchange with 1H MAS NMR. Samples in the sealed glass
tubes were kept at room temperature prior to NMR analysis.
2.2. NMR Measurements. NMR spectra were recorded at
9.4 T on a Bruker Avance 400 spectrometer equipped with a
broadband double-resonance-MAS probe. 1H MAS NMR
spectra were recorded by the Hahn-echo pulse sequence (π/
2−τ−π−τ−acquisition), where τ equals one rotor period (333
μs). The length of excitation π/2 pulse was 5.0 μs, and typically
16 scans were accumulated with a 10 s delay. 27Al and 29Si MAS
NMR spectra were acquired under conditions similar to those
described earlier.7 The sample temperature was controlled by a
Bruker BVT- 1000 variable-temperature unit. The calibration of
the temperature (373−573 K) inside the rotor for kinetic
measurements was performed according to the protocol
described in refs 30 and 32.
Prior to acquisition of the signal at the kinetic measurements
of the H/D exchange, the NMR probe with the sample was
preheated for 20 min at the temperature at which the H/D
exchange did not yet occur at notable rate. This temperature
was 470 K for H-ZSM-5 and In+/H-ZSM-5 and 410 K for
InO+/H-ZSM-5 zeolite samples. Then the temperature was
3. RESULTS AND DISCUSSION
The In-modified zeolite ZSM-5 (In/H-ZSM-5), prepared by
reductive solid-state ion exchange between indium(III) oxide
and H-ZSM-5 zeolite, contains indium species either in the
form of In+ or InO+ cations. In/H-ZSM-5 treated with
hydrogen at 773 K produces In+/H-ZSM-5 zeolite containing
indium in the form of In+ cations. Treatment of In+/H-ZSM-5
with oxygen at 623 K gives the zeolite InO+/H-ZSM-5, in
which indium exists in the form of InO+ cations.7 Both In+/HZSM-5 and InO+/H-ZSM-5 zeolite samples were tested in the
reaction of H/D hydrogen exchange with methane-d4 (CD4).
Monitoring H/D exchange between the acid OH groups and
CD4 with 1H MAS NMR in situ for In+/H-ZSM-5 shows an
increase of the intensity of proton signal of methane CHnD4−n
(n = 1−4) at 0.1 ppm in the 1H MAS NMR spectrum of CD4
adsorbed on the zeolite (Figure 1). Increase of the intensity of
Figure 1. Stack plot of 1H MAS NMR spectra at 568 K of methane-d4
adsorbed on In+/H-ZSM-5 zeolite. The first spectrum (bottom) was
recorded 6 min after and the last spectrum (top) 500 min after the
temperature was raised to 568 K. The time between subsequent
spectra recording was 10 min.
the signal from methane with time is indicative of H/D
exchange reaction, which occurs by a transfer of protium from
the acid OH group of the zeolite (BAS) to the deuterated
methane and simultaneous reverse transfer of deuterium from
methane-d4 to the zeolite OH groups. Analysis of the kinetics of
H/D exchange (Figure 2) shows that exchange occurs in the
temperature range of 540−570 K for In+/H-ZSM-5, i.e., similar
to the temperature range of the exchange reaction of methane
with BAS of H-ZSM-5 zeolite.29 The activation energies of the
reactions are also similar (within experimental error): 127 ± 27
and 118 ± 9 kJ mol−1 for In+/H-ZSM-5 and H-ZSM-5,
respectively. However, the rate of the exchange for In+/HZSM-5 is almost 1 order of magnitude larger than that for the
exchange of methane on the pure acid form zeolite H-ZSM-5
(see Figure 3). This indicates that indium existing in the zeolite
in the form of In+ cations accelerates the H/D exchange
reaction.
For InO+/H-ZSM-5 zeolite, the temperature threshold of the
reaction is 90 K lower than that for In+/H-ZSM-5 zeolite; the
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Figure 2. Experimental kinetics for H/D exchange of methane-d4 on In+/H-ZSM-5 zeolite. The rate constants shown nearby each kinetic curve were
calculated based on exchange model described in refs 19, 28, 30, and 33.
Figure 4. Stack plot of 1H MAS NMR spectra at 455 K of methane-d4
adsorbed on InO+/H-ZSM-5 zeolite. The first spectrum (bottom) was
recorded 5 min after and the last spectrum (top) 1000 min after the
temperature was raised to 455 K. The time between subsequent
spectra recording was 30 min.
Figure 3. Arrhenius plots for the H/D exchange reaction of methaned4 on H-ZSM-5 (Ea = 118 ± 9 kJ mol−1; ∇) (the data are adopted
from ref 29), In+/H-ZSM-5 (Ea = 127 ± 27 kJ mol−1; Δ), and InO+/
H-ZSM-5 (Ea = 74 ± 6 kJ mol−1; ○). Arrhenius plot for the reaction of
methane conversion to indium methyl species CH3InO (Ea = 74 ± 8
kJ mol−1; □).
methane on InO+/H-ZSM-5 (Figure 5) have been further
analyzed in terms of two parallel reactions depicted in Scheme
1: the H/D exchange reaction by pathway 1 and the conversion
to the indium methyl species CH3InO by pathway 2 (details of
kinetics scheme for deriving kinetic parameters are provided in
Supporting Information). Analysis of the kinetics within a
frame of Scheme 1 has shown that the rates of H/D exchange
(described by rate constant ka) and methane transformation to
CH3InO species (described by the rate constant kb) are similar
at the studied temperature range. The activation energies for
these reactions, derived from Arrhenius plots (Figure 3), are of
the similar values (74 kJ mol−1).
The kinetic parameters derived show that while the H/D
exchange occurs on In+/H-ZSM-5 1 order of magnitude faster
than that on H-ZSM-5, the rate of the exchange on InO+/HZSM-5 is 2 orders of magnitude larger than the rate on HZSM-5 (see Figure 3). The activation energy of the exchange
exchange occurs at 450−520 K. Figure 4 shows the evolution of
1
H MAS NMR spectrum of CD4 adsorbed on InO+/H-ZSM-5
with time. The intensity of the proton signal of CHnD4−n
increases for the first 250 min of the reaction at 455 K, but with
longer times the intensity of the signal decreases (Figures 4 and
5). A volcano shape of the kinetic curve is related to two
processes occurring on the zeolite after CD4 adsorption. The
first process is the H/D exchange reaction of methane with
BAS, and the second one is the chemical reaction of methane
with InO+ cations to afford indium methyl species CH3InO.
The occurrence of the latter reaction has been demonstrated
recently.7 Although the conversion of methane into CH3InO
species is notable, no signal from the CH3InO (−1.7 ppm)7
was observed in the spectra (Figure 4). This might be due to an
extremely large width of this signal.7 The kinetic curves for
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Figure 5. Kinetics of H/D hydrogen exchange for methane-d4 on InO+/H-ZSM-5 zeolite. The rate constants ka and kb correspond to pathways 1 and
2 in Scheme 1
One can suggest that the promoting effect of both In+ and
InO+ cations on the reaction of the H/D exchange is related to
involvement of the cationic species by forming some transient
complex with methane, which can further participate in H/D
exchange with BAS of the In-modified ZSM-5 zeolites. The
stronger effect of InO+ species on the kinetic parameters of the
exchange might indicate that the complex of methane with
InO+ species is more effective in terms of activation of methane
molecules.
The significant acceleration of H/D exchange, coupled with
the decreased activation energy on InO+/H-ZSM-5 compared
to that on In+/H-ZSM-5 zeolite, demonstrates that the
reversible cleavage and formation of the C−H(D) bonds in
methane occurs with InO+ species easier than with In+ species.
Moreover, InO+ not only transfers H and D atoms between the
methane and BAS of the zeolite but also participates in the
chemical reaction, as has been shown before:7
Scheme 1. Possible Structure for Transient Complex I of
Methane Interacting with BAS and InO+ Species of InO+/HZSM-5 Zeolite and Pathways of the Complex Evolution
toward Either the H/D Exchange or the Transformation to
Indium Methyl Species CH3−InOa
CH4 + InO+ZO− → CH3InO + ZOH
(1)
−
where ZO denotes the zeolite framework.
It is reasonable to assume that the interaction of methane
with InO+/H-ZSM-5 gives rise to the formation of some
transient complex I as shown in Scheme 1. Formation of this
complex implies that Al pairs should be located at appropriate
distance from each other. For Si/Al = 17, statistical evaluation
of Al distribution in zeolite framework predicts that the zeolite
sample used in this work might contain about 30% of Al−Si−
Si−Al pairs,34 whereas recent experiments show that the
quantity of such pairs can reach 50%.35 So the formation of
complex I with involvement of Al−Si−Si−Al pairs is quite
possible. This complex I might be further involved in pathway
1, leading to the H/D exchange reaction with another acidic
OH group of the zeolite. Or this complex might follow pathway
2 to form indium-methyl species CH3InO. We believe that the
oxygen-containing InO+ species facilitates these reactions better
than the same processes occurring with involvement of In+
cations only.
a
H/D exchange of methane on InO+/H-ZSM-5 zeolite is described by
the rate constant ka, and concurrent methane conversion to CH3−
InO species is described by the rate constant kb.
reaction is smaller by 50 kJ mol−1 for InO+/H-ZSM-5. These
results indicate that both In+ and InO+ cationic species facilitate
the H/D exchange reaction. However, the effect of InO+
species on H/D exchange reaction is more striking, providing
an essential increase of the rate and decrease of apparent
activation energy and leading to the decrease of the
temperature threshold for reaction proceeding by almost 90 K.
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of Ethylene over Metal Cations-Loaded H-ZSM-5. Appl. Catal., A
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(6) Baba, T.; Abe, Y.; Nomoto, K.; Inazu, K.; Echizen, T.; Ishikawa,
A.; Murai, K. Catalytic Transformation of Methane over In-Loaded
ZSM-5 Zeolite in the Presence of Ethene. J. Phys. Chem. B 2005, 109,
4263−4268.
(7) Gabrienko, A. A.; Arzumanov, S. S.; Moroz, I. B.; Prosvirin, I. P.;
Toktarev, A. V.; Wang, W.; Stepanov, A. G. Methane Activation on InModified ZSM-5: The State of Indium in the Zeolite and Pathways of
Methane Transformation to Surface Species. J. Phys. Chem. C 2014,
118, 8034−8043.
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(10) Kramer, G. J.; van Santen, R. A.; Emeis, C. A.; Nowak, A. K.
Understanding the Acid Behavior of Zeolites from Theory and
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Santen, R. A. Density Functional Theory Calculations of the
Transition States for Hydrogen Exchange and Dehydrogenation of
Methane by a Brønsted Zeolitic Proton. J. Phys. Chem. 1994, 98,
12938−12944.
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Exchange Between CD4 and the H-Forms of Zeolites FAU and MFI. J.
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Zeolite from Generalized Valence-Bond Plus Configuration Interaction Calculations. J. Mol. Struct.: THEOCHEM 1996, 371, 237−243.
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Exchange. J. Phys. Chem. B 1999, 103, 10417−10420.
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Kinetics of Hydrogen−Deuterium Exchange Reactions of Methane
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Study of H/D Isotope Exchange Reaction of Alkanes and Brønsted
Acidic Hydroxyl Groups of FER Zeolite. Catal. Lett. 1999, 59, 51−54.
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Thus, our experiments demonstrate a synergistic action
between the exchange cations and the BAS for activating
methane on In-modified zeolites. Presumably a similar
mechanism is valid on other metal-modified zeolites, loaded
for example with cations of Zn, Ga, or Ag.28−30,36−40
4. CONCLUSION
We have performed a comparative analysis of the kinetics of
hydrogen (H/D) exchange between methane-d4 and Brønsted
acid sites for both the pure acid form zeolite (H-ZSM-5) and
In-modified zeolites (In+/H-ZSM-5 and InO+/H-ZSM-5) with
1
H MAS NMR spectroscopy in situ within the temperature
range of 453−568 K. The analysis shows that the rate of
exchange is 1 order of magnitude larger on In+/H-ZSM-5 and 2
orders of magnitude larger on InO+/H-ZSM-5 than that on HZSM-5. Besides dramatic increase of the rate, a notable
decrease of the activation energy (Ea = 74 ± 6 kJ mol−1) and
the temperature threshold (453 K) for the reaction of the
exchange on InO+/H-ZSM-5 compared to the rate, activation
energy (Ea = 127 ± 27 kJ mol−1), and temperature threshold
(543 K) for the reaction on In+/H-ZSM-5 and H-ZSM-5 has
been detected. The significant change of the kinetic parameters
of the exchange for InO+/H-ZSM-5 is rationalized in terms of
involvement of both InO+ species and BAS in activation of
methane molecules on this In-modified zeolite. Some transient
intermediate complex of methane with the zeolite InO+ species
and BAS is assumed to be formed within the void of the zeolite
pore. This complex is either involved in the reaction of H/D
exchange with BAS of the zeolite or evolves further to offer
indium methyl species.
■
ASSOCIATED CONTENT
S Supporting Information
*
Description of kinetics scheme for deriving kinetic parameters.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*Tel: +7 952 905 9559. Fax: +7 383 330 8056. E-mail:
[email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors are grateful to Dr. A.V. Toktarev for the synthesis
of In-modified zeolite. This work was supported in part by
Russian Foundation for Basic Research (Grant 14-03-00040).
■
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NOTE ADDED AFTER ASAP PUBLICATION
This paper was published to the Web on 6/19/2014, with a
minor error in the Introduction. This was fixed in the version
published on 6/23/2014.
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